Assuming you've already done the basic checks for malware, have you done any troubleshooting for Firefox itself? I've been using Firefox for over a decade and have never had any not-easily-fixable trouble with it. It sounds like you're using at least a few add-ons. You can launch Firefox in 'safe mode', which disables all your add-ons. If you don't have problems while using safe mode, then one of your add-ons probably updated and messed itself up (or failed to update, and is no longer as compatible with a recent update of Firefox).
If you DO still have problems, it may be your profile. You can either simply clear all your web data ("Clear recent history", clear all), or launch the Profile manager (from a command prompt do 'firefox -p') and just make a new profile. (The profile manager is a nice, sort of 'hidden feature' of Firefox. My wife and I can now both use my laptop and not destroy each other's saved tabs).
I probably have over 100 tabs (oops, should probably clean that out) open over four tab groups in Firefox and have no lag issues. I use Firefox across Ubuntu, Windows, and Android and it works great on all of them, and they're all using Sync so I can easily open a tab I had open on, say, my work laptop after I get home.
Chrome can save your preferences, bookmarks, passwords, etc. as a profile that you can access from any device with Chrome
Firefox can do this, too, if you bother to take five seconds to set up Sync. And as a bonus, it lacks the shadiness of Google. You can even set up your own server to hold the Sync information if you feel like figuring out how to build the software they use. That's the beauty of open source software.
EDIT: Also, if you're not already, use the add-on Ghostery. Everyone should be using this add-on. It blocks all the stupid crap that sites use to track you (facebook buttons, G+ buttons, third party ad tracker cookies, etc.) and serve irritating ads and pop-ups to you, while not entirely blocking all advertising like adblock pro does (so sites that use non-invasive advertising can still make money off of it).