The more extensions you have, the more resources it takes up. If you're concerned with memory, try the following tweaks:Reduce the amount of RAM Firefox uses for its cache feature
1. Type “about:config” (no quotes) in the adress bar in the browser.
2. Find “browser.sessionhistory.max_total_viewer”
3. Set it’s value to “0“Reduce RAM usage to 10MB when Firefox is minimized
1. Open Firefox and go to the Address Bar. Type in about:config and then press Enter.
2. Right Click in the page and select New -> Boolean.
3. In the box that pops up enter “config.trim_on_minimize”. Press Enter.
4. Now select True and then press Enter.
5. Restart Firefox.
As for add-ons like Speedfox, it's up to you to try it. I've heard these type of add-ons can cause compatibility problems. Here's a couple of comments I read:
All the config changes you can do yourself in under 5 minutes. The Facebook re-direct was bad enough but when I discovered what little changes it made, I just went to About Config and set up my own settings which result in a damn faster Firefox than this terrible extension. Remove all the Facebook garbage and be honest about the config changes it makes and it might be at least decent but even then, it is still totally and completely unnecessary.
This thing slowed down FF so much for me that I was about to re-install an older version of the browser, thinking that was the problem. Luckily, I remembered I had recently installed this add-on. Sure enough, I disabled it and everything went back to normal.