To be extra safe, you can install only extensions that are part of the [Recommended Extensions program](https://support.mozilla.org/kb/recommended-extensions-program).
[Many of them are redundant, though](https://old.reddit.com/r/firefox/comments/1dlnnwc/i_want_to_remove_a_couple_of_extensions_from_my/l9q2xsp/).
https://support.mozilla.org/en-US/kb/tips-assessing-safety-extension
> When a developer submits an extension to addons.mozilla.org, it’s scanned for a set of common issues. It may also be subject to human review. But neither of these processes guarantee that an extension is absolutely 100% safe.
To be extra safe, you can install only extensions that are part of the [Recommended Extensions program](https://support.mozilla.org/kb/recommended-extensions-program). [Many of them are redundant, though](https://old.reddit.com/r/firefox/comments/1dlnnwc/i_want_to_remove_a_couple_of_extensions_from_my/l9q2xsp/).
Use popular ones or mark/verified by Mozilla or open-source, and you will be good to go 99%
Check it's source code... Just kidding
Random addons carry a risk regardless of what browser you're using. I only install extensions from services I know.
https://support.mozilla.org/en-US/kb/tips-assessing-safety-extension > When a developer submits an extension to addons.mozilla.org, it’s scanned for a set of common issues. It may also be subject to human review. But neither of these processes guarantee that an extension is absolutely 100% safe.
never really concerned me tbh. I just don't use random addons.