The extension is very useful for people who are curious about what font a page is using. However, my problem is that it slows Firefox down dramatically on my computer (Fedora 22, Firefox 39, 2.2 GHz dual-core with 2 GB RAM). I know, not a high-end machine, but without this extension, everything is pretty snappy. I use several other extensions and none of the others have this problem.
This is the simplest of all extensions with barely 50 lines of code and only has access to webmail sites - so it's probably not the culprit or you would see many more reviews like this one.
