Move to Chrome. The biggest thing it will give you is a clean profile: no cache, no addons, no massive history. Make sure you delete all your Firefox info before you move over.
Then when Chrome starts to feel slow go back to Firefox.
I think the problem with Firefox (at least for me) is being caused by the function they built in that lets you see which sites are trying to track you.
I use Firefox. I have had moments of outrage, but that was on an older machine. On my new machine it works a dream. Really there is no reason your browser should be slow to load, you should consider that your 'puter is just crap.
I use Opera, and have done for the past 3 years or so, lovely browser, very rarely does it let me down, and has a lovely front page that's fully customized