User-Agent: Mozilla/5.0 (Macintosh; U; Intel Mac OS X 10_6_4; en-us) AppleWebKit/533.18.1 (KHTML, like Gecko) Version/5.0.2 Safari/533.18.5 Build Identifier: Mozilla/5.0 (Macintosh; U; Intel Mac OS X 10.6; en-US; rv:184.108.40.206) Gecko/20100914 Firefox/3.6.10 I downloaded firesheep.xpi, and double-clicked on it. Firefox did not register itself as a handler for .xpi files, so I had to switch from "Recommended Applications" to "All Applications" in the app picker, and then select Firefox. Reproducible: Always Steps to Reproduce: Double-click a .xpi file. Actual Results: Mac does not know how to handle it. Expected Results: Mac should pass the .xpi file to Firefox.
Workaround: Drag the link directly from the page to the Tools -> Add-ons dialog.
Is this a duplicate of bug 313468 or not(being a different OS)?
It looks like bug 313468 is conceptually the same problem, but the fix will be entirely different code/configuration on Mac vs. Windows, so I would avoid marking one as a duplicate. But yes, that discussion is relevant here. The full workaround is to open an .xpi file, choose the option to specify a helper, click App Applications, select Firefox. Or a similar path via the Finder's Get Info window. And there is no conflict with Thunderbird, fortunately. The Finder is smart enough to handle 2 different available helpers for a type.
WFM, due to the fact that the workaround from comment 1 is available.