Mozilla has released an updated version, 5.0.1, of its Firefox browser for Mac OS X, Linux, and Windows. The new version incorporates fixes for a couple of problems specific to the Mac platform; further details are in the Release Notes.
You can obtain the new version, for all platforms (Mac OS X, Linux, and Windows), in 70+ languages, from the download page. Alternatively, you can use the built-in update mechanism (Help -> About Firefox -> Check for Updates).