> I stick to extensions that Mozilla has manually vetted as part of the Firefox recommended extensions program.

If you're feeling extra-paranoid, the XPI file can be unpacked (ZIP) and to check over the code for anything suspicious or unreasonably-complex, particularly if the browser-extension is supposed to be something simple like "move the up/down vote arrows further apart on HN". :P

While that doesn't solve the overall ecosystem issue, every little bit helps. You'll know it's time to run away if extensions become closed-source blobs.

You can also, more conveniently, plug an extension's URL into this viewer:

https://robwu.nl/crxviewer/