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The sites I try to install the extensions from are allowed in:
options/web features/Allow web sites to install software/
I even tried to remove Firefox, installed it back and it changed nothing!
If you still have trouble after that, and if you have upgraded Firefox without uninstalling the previous version first, then I suggest that you uninstall Firefox and re-install it.
This has not been so much of a problem recently, but before the 1.0 release it was often necessary to uninstall the old version before reinstalling the new.
Uninstalling Firefox does not delete your profile folder, so bookmarks and preferences should not be affected by the uninstall/reinstall. If in doubt, make a back-up.