Firefox 3.0.18 fixes the following issues found in previous versions of Firefox 3:
MFSA 2010-05 XSS hazard using SVG document and binary Content-Type MFSA 2010-04 XSS due to window.dialogArguments being readable cross-domain MFSA 2010-03 Use-after-free crash in HTML parser MFSA 2010-02 Web Worker Array Handling Heap Corruption Vulnerability MFSA 2010-01 Crashes with evidence of memory corruption (rv:1.9.1.8/ 1.9.0.18)
See the Firefox 3.0.17 release notes for changes in previous releases.
The following is quoted from the "Extensions and Themes" section in the release notes at the 2nd link below: ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Extensions and Themes
Extensions installed under Firefox 2 may be incompatible and/or require updates to work with Firefox 3. Please report any issues to the maintainer of the extension. When you install Firefox 3 all of your Extensions and Themes will be disabled until Firefox 3 determines that either a) they are compatible with the Firefox 3 release or b) there are newer versions available that are compatible. ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~