I am using the trunk version of firefox a lot because it’s much faster than the current official version.
Recently the preference to enable old versions of extensions in firefox has been changed:
has been renamed to
With the current version being 3.7a or new 4.0b the preference key is
Setting this preference key to the boolean value false will allow you to enable add-ons that are not tagged as compatible with the current development version of firefox. Of course because of possible API changes, especially in Version 4 some plugins may be broken, but for me all add-ons I use still work on the nightly builds.