1) That may depend on where you obtained that extension. But overall very doubtful that is could cause long term problems, with the current "WebExtensions" format that Firefox is using since Firefox 57 Quantum was released in November 2017.

2) Very doubtful, plus any Firefox WebExtension from anywhere would need to be "digitally signed" in order for Firefox to even install it. And that "signing" process is carefully controlled by Mozilla. Plus Mozilla can disabled any WebExtension at any time if that extension is found to be malicious or found to be "spyware"; after the fact. A 'check' for such is done everytime that Firefox is opened to be used.

And with Mac OS's that would be very a very rare occurrence.

I'm talking about this particular extension here:

https://addons.mozilla.org/en-US/firefox/addon/almonit/

what concerns me is the following:

1) There is message underneath "This is not monitored for security through Mozilla's Recommended Extensions program. Make sure you trust it before installing."

2) There are only 2 positive reviews

3) All people who are supposedly be key developers have broken links to their github.

4) all key developers listed on extension page joined very recently

5) if you open github itself I have to admit, that some work is done, but that does not mean that, the extension itself is harmless

does it change your opinion in the light of that information?