Installing adobe flash

The symptoms you're describing are weird (I've never seen them before, and with ~2.5 billion users, that's saying a lot).  Weird usually means something low-level, like the underlying hardware or storage is freaked out.  When higher level stuff like application software is running on a machine and the nuts and bolts stuff like memory and disk access isn't behaving perfectly, you're going to get weird symptoms.  Your description is screaming that.

If you haven't rebooted the computer, that's probably a pretty great place to start.  Actually shut it down, let it sit for 15-30 seconds so that everything fully discharges, then fire it back up.

After that, if you re-launch Firefox and the problem persists, open up Task Manager and sort by CPU.  Is there something running that's consuming a ton of CPU cycles?  I'm wondering if maybe the uninstallation of McAfee failed and it's still trying to do an initial scan of the machine.  If that's the case, you'll probably see at the top of the list if you're looking at CPU usage.

If rebooting doesn't help and nothing is causing CPU contention, then I'd be curious about why Firefox is crashing.  My thinking is that the next thing that would be super slow is storage, particularly if Firefox is trying to read and write from a cache, and those reads or writes are failing.  There are probably multiple retries happening at various levels, which would make it really slow to do anything, if it's consistently hitting problems accessing files on the disk.

If you go to about:crashes in Firefox and click the first couple links, that will submit them to Mozilla.  If you copy and paste the resulting links here, I'd be happy to take a look (if you can even get that far)

My guess is that there's a really good chance that the machine is just freaked out and that actually cold-booting the machine will solve it.  "When in doubt, reboot" is still sage advice.  If that doesn't help, I'm thinking that you're seeing symptoms of an underlying problem, like the filesystem is corrupt and/or some system files are damaged, that kind of thing.  There's a strong correlation to the installation/uninstallation activity because a lot of activity happened.  If the filesystem or registry was in a bad state and then you did a bunch of writes and removals, you get a big mess that doesn't work right.

If you can't get the crash dumps, (and I'm assuming this is Windows 7 or below), then you might just try running the disk repair utility to see what you find. 

Also, if the actual disk hardware *is* failing (it would look a lot like this initially), scanning and repairing errors is going to put the disk through a workout.  If you have any critical data on the machine that you haven't backed up, I'd strongly recommend doing that first.  At that point if the disk dies, at least you're looking at < $100 for a replacement hard disk, and not > $1500 for a fancy data recovery service.