There are a couple issues related to Firefox 3.6 & 5.0 and Mac OSX 10.7 as described on the Mozilla Product’s Blog:

• There is a bug present in the upcoming release of OSX 10.7 that causes Firefox to crash when using downloadable fonts.  Downloadable fonts are used widely on the web, so users would have suffered severe crash problems.  We alerted Apple to the problem before the release of 10.7 but they did not fix the problem before 10.7 went to final release.  We’ve changed the font APIs that we’re using to newer versions which appear to fix the problem.  Users on 10.6 and before will continue to use the old APIs to reduce risk.  Later versions of Firefox will move all 10.6 and later users to the new APIs.
• Apple recently updated Java on OSX 10.5 and it broke the Java plugin in Firefox.  Although the number of people using Firefox on 10.5 is quite small, this update fixes Firefox so that it can load the Java plugin on 10.5.

As a result Mozilla will be releasing an emergency update to Firefox 5 for Mac users only. Firefox 3.6.19 & 5.0.1 should be release today (July 11th). There will not be a Firefox 3.6.19 or 5.0.1 for Windows, Linux or Mobile users. Further all planned patches/fixes for 3.6.19 have been moved to 3.6.20 due out on August 16th.