mi tocca di rispondermi da solo: stavo usando una vers. vecchia del browser:

Fix Bugs

Is there some bug that really bothers you? As well as reporting it, feel free to fix it. Fixing bugs in Mozilla is far easier than in many other applications, because you can fix bugs (such as those in our cross-platform front end, written in XUL, our XML-based User-interface Language, CSS and Javascript) using only the build you are running right now. There is no need to set up complex environments, or compile anything. We've even written a tool to make it really easy to get started - Patch Maker. So, if you've written web pages, you can hack on Mozilla.