Try clearing the cache. I once had the problem and clearing the cache solved it. Click here.

I agree Firefox RAM usage is ridiculous. This thing uses way more memory than FF on my PC. I dont get the slowdown unless there is some bad files in the cache. On some RARE occasions, you may have to clear data in android control panel for firefox to fix your slowdown. But DONT do this until you try clearing cache first. Because clearing data deletes all your bookmarks and saved passwords/cookies.