I'm running firefox 3.6.12 on mac os x 10.6.5. I started using HTTPS Everywhere and firefox became almost unusably slow. It started with some websites loading slowly, which I could imagine is a result of using https. But then even pages that were already loaded would be slow, firefox would hang when scrolling down the page, etc. I disabled HTTPS Everywhere and the problem went away. Is this just the plugin, or is it possible something else is going on?
If you're interested in seeing what it runs, the .xpi file you downloaded (if you re-download it to a folder) to install it can be simply renamed to .zip and unzipped for you to look at the contents, using the default Mac archive utility, or if you use 7-zip, you can just right-click it and extract to a folder for viewing.