See [[Installing a previous version of Firefox]]. For info, it is up to the authors of the extensions to update them to work with newer versions of Firefox.

Which extensions do not work with Firefox 3.6.13? Delete the files extensions.* (extensions.rdf, extensions.cache, extensions.ini, extensions.sqlite) and compatibility.ini in the Firefox [[Profiles|profile folder]] to reset the extensions registry. New files will be created when required.<br /> See "Corrupt extension files": http://kb.mozillazine.org/Unable_to_install_themes_or_extensions If you see disabled, not compatible, extensions in "Tools > Add-ons > Extensions" then click the "Find Updates" button (in Firefox 4: right-click the extension -> "Find Updates") to do a compatibility check. ---- There are other things that need attention: Your above posted system details show outdated plugin(s) with known security and stability risks that you should update. # Shockwave Flash 10.0 r45 # Next Generation Java Plug-in 1.6.0_16 for Mozilla browsers Update the [[Managing the Flash plugin|Flash]] plugin to the latest version. *http://www.adobe.com/software/flash/about/ Update the [[Java]] plugin to the latest version. *http://www.oracle.com/technetwork/java/javase/downloads/index.html (Java Platform: Download JRE)