The symptom remains that if I kill firefox and restart it, things run a
lot faster for a few hours, and then bog down again.
I believe that 4GB of RAM is now not enough for web browsing, especially
with web browsing involving _javascript_.
Whether you continue to use Firefox, if you can, I suggest that you
upgrade your RAM, to as much as your motherboard will take. Whilst I do
not know what the pricing of RAM is like, now, it was, a while ago,
quite inexpensive, and, a desktop computer that I bought, with an i3
CPU, was upgraded to 32GB of RAM, as soon as I could, after buying it,
and, it has run well with that.
MB has all the RAM it can take, so more RAM would require a new MB.