I would also like to know. After a bit of research (not really familiar with Linux directory structure...) I found hidden Mozilla folder ( ~/.mozilla/fennec/ ) and one directory deeper there's a 'extensions' directory. Under this extensions folder, there are few directories which I guess correspond my installed extensions. The directory names are mostly some random ID inside curly brackets (I guess those curly brackets block the autocomplete with tab so you have to write those long names by hand ).

Anyway, I edited all install.rdf files and changed every maxversion to 1.* (from 1.0.*) under <em:tagetApplication> but it didn't help. Weave is still the only one that works. All I need is Adblock. But I have to say that there nightly builds are so much better that it makes up the lack of extensions. Startup time is shorter, start page is better and browser feels snappier.