Mac Security: Firefox add-ons safe?

They are absolutely safe. The only ones that are possibly unsafe are ones marked as "not reviewed by Mozilla" (there aren't many of those).

 

Mozilla has a review process that includes automated tools that scan the add-ons as well as some peer reviewing. There was an incident where the scanning wasn't being done properly and a few bad ones got through, but that was the exception. Here's an example of one such time where an experimental add-on had a trojan that effect Windows users. And here's an example where a virus came through the Vietnamese language pack. These are very rare though and haven't impacted OSX as of yet.

So, yes they are considered very safe, though not always stable, but even that has gotten better over the years.

 

If you're so concerned about safety, read the add-on's comments. It's a popular add-on, so I wouldn't really worry.