I've been having this problem for a few days. It started right around the time 2 things happened: - we changed the way devtools resource URLs look like (see https://github.com/jryans/devtools-migrate/blob/master/resource-devtools/README.md) - I played with the gcli 'tools reload' command. Since then, every time I execute './mach build devtools', then none of the changes I made to my source files are loaded in firefox when I run it with './mach run -P dev'. I have to do a full './mach build' for it to work. Thinking this might have been related to me playing with the gcli command, I removed the srcdir pref and stopped using it. I'm on Windows 10. Let's take the example of this file: /devtools/client/animationinspector/components.js - I do a local change to the file in Sublime, - then I run './mach build devtools' (or even './mach build devtools/client', it has the same effect for me), - then I run firefox './mach run -P dev' and open the devtools I can see that my change wasn't taken into account. In fact, if I then open this URL in a new browser tab: resource://devtools/client/animationinspector/components.js I can see that my change isn't here. I kept on digging around a bit more and found out that if I execute this piece of code: let target = "resource://devtools/client/animationinspector/components.js"; let resourceHandler = Services.io.getProtocolHandler("resource") .QueryInterface(Ci.nsISubstitutingProtocolHandler); let resURI = Services.io.newURI(target, null, null); let uri = Services.io.newURI(resourceHandler.resolveURI(resURI), null, null); then I get uri.spec = file:///c:/Users/Patrick/dev/fx-team/obj-x86_64-pc-mingw32/dist/bin/browser/chrome/devtools/modules/devtools/client/animationinspector/components.js Using the windows explorer (or a new tab in Firefox), I can see that this file does exist but doesn't have my changes. In fact, keeping an explorer window opened on the parent folder while running './mach build devtools' shows that this file isn't being overridden. If I run './mach build' then it does get overridden. One more thing, the file doesn't appear to be a symlink file to the actual source file (which is what I think happens on Linux). I'm not sure what are the next steps that I should take to investigate this further, but something is definitely wrong here.

I'll take a look.

I believe this is the same issue as bug 1219368, but let's leave this open until we're sure.

'./mach build faster' does work for me.

Great, I'll send a mail to update the team on this.