--- a/toolkit/crashreporter/client/app/src/config.rs
+++ b/toolkit/crashreporter/client/app/src/config.rs
@@ -365,50 +365,26 @@ impl Config {
     /// program bundle. On other platforms this assumes siblings reside in the same directory as
     /// the crashreporter.
     ///
     /// The returned path isn't guaranteed to exist.
     // This method could be standalone rather than living in `Config`; it's here because it makes
     // sense that if it were to rely on anything, it would be the `Config` (and that may change in
     // the future).
     pub fn sibling_program_path<N: AsRef<OsStr>>(&self, program: N) -> PathBuf {
-        // Expect shouldn't ever panic here because we need more than one argument to run
-        // the program in the first place (we've already previously iterated args).
-        //
-        // We use argv[0] rather than `std::env::current_exe` because `current_exe` doesn't define
-        // how symlinks are treated, and we want to support running directly from the local build
-        // directory (which uses symlinks on linux and macos).
-        let self_path = PathBuf::from(std::env::args_os().next().expect("failed to get argv[0]"));
+        let self_path = self_path();
         let exe_extension = self_path.extension().unwrap_or_default();
-
-        let mut program_path = self_path.clone();
-        // Pop the executable off to get the parent directory.
-        program_path.pop();
-        program_path.push(program.as_ref());
-        program_path.set_extension(exe_extension);
-
-        if !program_path.exists() && cfg!(all(not(mock), target_os = "macos")) {
-            // On macOS the crash reporter client is shipped as an application bundle contained
-            // within Firefox's main application bundle. So when it's invoked its current working
-            // directory looks like:
-            // Firefox.app/Contents/MacOS/crashreporter.app/Contents/MacOS/
-            // The other applications we ship with Firefox are stored in the main bundle
-            // (Firefox.app/Contents/MacOS/) so we we need to go back three directories
-            // to reach them.
-
-            // 4 pops: 1 for the path that was just pushed, and 3 more for
-            // `crashreporter.app/Contents/MacOS`.
-            for _ in 0..4 {
-                program_path.pop();
-            }
-            program_path.push(program.as_ref());
-            program_path.set_extension(exe_extension);
+        if !exe_extension.is_empty() {
+            let mut p = program.as_ref().to_os_string();
+            p.push(".");
+            p.push(exe_extension);
+            sibling_path(p)
+        } else {
+            sibling_path(program)
         }
-
-        program_path
     }
 
     cfg_if::cfg_if! {
         if #[cfg(mock)] {
             fn get_data_dir(&self, vendor: &str, product: &str) -> anyhow::Result<PathBuf> {
                 let mut path = PathBuf::from("data_dir");
                 path.push(vendor);
                 path.push(product);
@@ -503,16 +479,62 @@ impl Config {
                 path.push(product);
                 path.push("Crash Reports");
                 Ok(path)
             }
         }
     }
 }
 
+/// Get the path of a file that is a sibling of the crashreporter.
+///
+/// On MacOS, this assumes that the crashreporter is its own application bundle within the main
+/// program bundle. On other platforms this assumes siblings reside in the same directory as
+/// the crashreporter.
+///
+/// The returned path isn't guaranteed to exist.
+pub fn sibling_path<N: AsRef<OsStr>>(file: N) -> PathBuf {
+    // Expect shouldn't ever panic here because we need more than one argument to run
+    // the program in the first place (we've already previously iterated args).
+    //
+    // We use argv[0] rather than `std::env::current_exe` because `current_exe` doesn't define
+    // how symlinks are treated, and we want to support running directly from the local build
+    // directory (which uses symlinks on linux and macos).
+    let dir_path = {
+        let mut path = self_path();
+        // Pop the executable off to get the parent directory.
+        path.pop();
+        path
+    };
+
+    let mut path = dir_path.join(file.as_ref());
+
+    if !path.exists() && cfg!(all(not(mock), target_os = "macos")) {
+        // On macOS the crash reporter client is shipped as an application bundle contained
+        // within Firefox's main application bundle. So when it's invoked its current working
+        // directory looks like:
+        // Firefox.app/Contents/MacOS/crashreporter.app/Contents/MacOS/
+        // The other applications we ship with Firefox are stored in the main bundle
+        // (Firefox.app/Contents/MacOS/) so we we need to go back three directories
+        // to reach them.
+        path = dir_path;
+        // 3 pops for `crashreporter.app/Contents/MacOS`.
+        for _ in 0..3 {
+            path.pop();
+        }
+        path.push(file.as_ref());
+    }
+
+    path
+}
+
+fn self_path() -> PathBuf {
+    PathBuf::from(std::env::args_os().next().expect("failed to get argv[0]"))
+}
+
 fn env_bool<K: AsRef<OsStr>>(name: K) -> bool {
     std::env::var(name).map(|s| !s.is_empty()).unwrap_or(false)
 }
 
 fn env_path<K: AsRef<OsStr>>(name: K) -> Option<PathBuf> {
     std::env::var_os(name).map(PathBuf::from)
 }
 

--- a/toolkit/crashreporter/client/app/src/lang/omnijar.rs
+++ b/toolkit/crashreporter/client/app/src/lang/omnijar.rs
@@ -1,29 +1,31 @@
 /* This Source Code Form is subject to the terms of the Mozilla Public
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 use super::language_info::LanguageInfo;
+use crate::config::sibling_path;
 use crate::std::{
-    env::current_exe,
     fs::File,
     io::{BufRead, BufReader, Read},
     path::Path,
 };
 use anyhow::Context;
 use zip::read::ZipArchive;
 
 /// Read the appropriate localization fluent definitions from the omnijar files.
 ///
 /// Returns (locale name, fluent definitions).
 pub fn read() -> anyhow::Result<LanguageInfo> {
-    let mut path = current_exe().context("failed to get current executable")?;
-    path.pop();
-    path.push("omni.ja");
+    let mut path = sibling_path(if cfg!(target_os = "macos") {
+        "../Resources/omni.ja"
+    } else {
+        "omni.ja"
+    });
 
     let mut zip = read_omnijar_file(&path)?;
     let locales = {
         let buf = BufReader::new(
             zip.by_name("res/multilocale.txt")
                 .context("failed to read multilocale file in zip archive")?,
         );
         let line = buf
@@ -47,21 +49,26 @@ pub fn read() -> anyhow::Result<Language
     };
 
     // The brand ftl is in the browser omnijar.
     path.pop();
     path.push("browser");
     path.push("omni.ja");
 
     let ftl_branding = 'branding: {
-        for locale in &locales {
-            match read_branding(&locale, &mut zip) {
-                Ok(v) => break 'branding v,
-                Err(e) => log::warn!("failed to read branding from omnijar: {e:#}"),
+        match read_omnijar_file(&path) {
+            Ok(mut zip) => {
+                for locale in &locales {
+                    match read_branding(&locale, &mut zip) {
+                        Ok(v) => break 'branding v,
+                        Err(e) => log::warn!("failed to read branding from omnijar: {e:#}"),
+                    }
+                }
             }
+            Err(e) => log::warn!("failed to read browser omnijar: {e:#}"),
         }
         log::info!("using fallback branding info");
         LanguageInfo::default().ftl_branding
     };
 
     Ok(LanguageInfo {
         identifier: locale,
         ftl_definitions,

--- a/toolkit/crashreporter/client/app/src/std/env.rs
+++ b/toolkit/crashreporter/client/app/src/std/env.rs
@@ -35,11 +35,12 @@ pub fn var_os<K: AsRef<OsStr>>(_key: K) 
 }
 
 pub fn args_os() -> ArgsOs {
     MockCurrentExe.get(|r| ArgsOs {
         argv0: Some(r.clone().into()),
     })
 }
 
+#[allow(unused)]
 pub fn current_exe() -> std::io::Result<super::path::PathBuf> {
     Ok(MockCurrentExe.get(|r| r.clone().into()))
 }