I know you mentioned that you don't want to use another browser(though personally I must question why you keep using Firefox), but can you try Google Chrome and see if you experience a similar problem?
If you do, then it could be your computer or network, though if it's network, I can't help much, as I'm not too good on networking.
If not, then perhaps there's something that's gone amiss with your Firefox installation. You could do one of these:
-If you do want to keep a similar experience, try installing Waterfox. Waterfox is simply a 64-bit version of Firefox, allowing it to utilize more resources of your computer if need be. Waterfox tends to work better with certain things, so perhaps that would help.
-If you really want to stick to just Firefox, try uninstalling and reinstalling default, or wiping modifications made to it(set it to default).
I'll be here to help you the best I can.