Firefox keeps getting slower and slower as the day goes on

Question

It's exactly as the title suggests - it works fine first thing in the morning, then gradually gets slower as the day goes on. Last night, for example, I couldn't get on any sites for love nor money - all it kept saying was 'transferring to...' 'loading...' whatever site I was going on, YouTube videos keep freezing etc (I have to keep pausing and waiting for the little grey bar to extend a little; such a trivial thing, yet so annoying! I also unchecked the 'Use hardware acceleration where possible' thing in 'Options', to no avail).
I've done all the usual tests (scan, virus checks etc), all of which have come back perfectly clear. I used Internet Explorer, yet that works perfectly OK, aside from a little slowness in getting a page to load, but I think I'm using a rather dated version of IE (I got my computer September 2012, and this version of IE was already installed on there!), so maybe that's to blame.
I've reset Firefox, to no avail. It's getting on my nerves!! The YouTube part of my problem only started Wednesday night, and the slowness of Firefox started yesterday. As bizarre as it may seem, may the weather be causing the Internet to go slow, as it is unbelievably hot and humid here in the UK at the moment! I remember my Mum saying she had the same problem around this time last year when it was boiling hot!
Any help is greatly appreciated.

Answer 1

'''[https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-using-safe-mode Start Firefox in Safe Mode]''' {web link}
While you are in safe mode;
Press the '''Alt''' or '''F10''' key to bring up the tool bar.
Followed by;
Windows; '''Tools > Options'''
Linux; '''Edit > Preferences'''
Mac; ''application name'' '''> Preferences'''
Then '''Advanced > General.'''
Look for and turn off '''Use Hardware Acceleration'''.
Poke around safe web sites. Are there any problems?. Then restart.