--- a/browser/devtools/framework/ToolboxProcess.jsm
+++ b/browser/devtools/framework/ToolboxProcess.jsm
@@ -198,16 +198,27 @@ BrowserToolboxProcess.prototype = {
 
     if (this._options.addonID) {
       xulURI += "?addonID=" + this._options.addonID;
     }
 
     dumpn("Running chrome debugging process.");
     let args = ["-no-remote", "-foreground", "-profile", this._dbgProfilePath, "-chrome", xulURI];
 
+    // During local development, incremental builds can trigger the main process
+    // to clear its startup cache with the "flag file" .purgecaches, but this
+    // file is removed during app startup time, so we aren't able to know if it
+    // was present in order to also clear the child profile's startup cache as
+    // well.
+    //
+    // As an approximation of "isLocalBuild", check for an unofficial build.
+    if (!Services.appinfo.isOfficial) {
+      args.push("-purgecaches");
+    }
+
     process.runwAsync(args, args.length, { observe: () => this.close() });
 
     this._telemetry.toolOpened("jsbrowserdebugger");
 
     dumpn("Chrome toolbox is now running...");
     this.emit("run", this);
   },