It seems that you cannot install extensions when running Firefox 3 from a dmg. Repro steps: * Download Firefox 3 on a fresh mac from: http://www.mozilla.com/en-US/firefox/all-beta.html * Open DMG, but do not drag firefox icon to local machine * Double-click firefox icon inside DMG * Install, say, FlashBlock (https://addons.mozilla.org/en-US/firefox/addon/433) * I get this error: Firefox could not install the file at https://addons.mozilla.org/en-US/firefox/downloads/file/21584/flashblock-1.5.5-fx+ns+fl.xpi because: Unexpected installation error Review the Error Console log for more details. -203 It wasn't clear which error console it was referring to. I didn't see anything in the OS X system error console or Firefox's error console. Extension installation works correctly once Firefox icon is dragged to local machine and run from there. I also noticed that it *still* works after you delete the local firefox copy and run it again from the DMG.
Not enough information to proceed. Please reopen if you can follow the instructions in comment 1