Hi, ever since the october 24 catastrophe builds, it seems several bugs have been fixed, but these fixes are not showing up because old XUL.mfl files in user profile directories are not being automatically updated by mozilla with newer builds. I have seen A) middle click in message doesn't work and B) open tab in background doesn't work Fixed by deleting your XUL.mfl file. This file is regenerated when you start mozilla again.
If you are having a similar regression, try deleting your XUL.mfl file and reporting the results here. Thanks, Ben
