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Firefox Developer Edition and Beta: Try out Mozilla’s .deb package!
(hacks.mozilla.org)
A place to discuss the news and latest developments on the open-source browser Firefox
That's really handy actually, I'll have to keep it in mind, thanks!
See "What are my options if I want to install unsigned extensions in Firefox?" in the FAQ here: https://wiki.mozilla.org/Add-ons/Extension_Signing#FAQ
If you're on Linux, you may have that about:config flag even in your distro-provided Firefox Stable build...