I would like to add that whenever FF boots (to a blank page), without me hitting any websites, firefox.exe begins running, sending and receiving data immediately on booting as seen in Task manager. When I hit "Clear recent history" (active logins only), this often stops immediately and downloading my gmail account occurs about 10x faster under these conditions. But this only last for some period. Eventually, ff.exe begins another download(s) from somewhere, slowing me to a crawl, which for a dial-up is more like glacial velocity.

Firefox does make a number of connections and downloads when you open it and it may do that at other points while browsing. There are a number of things that may get updated while you use Firefox, such as the Google Safe Browsing database.

Most of these are small downloads that usually have little to no impact on the user's speed. However, if you have a slower internet connection, it's possible that you may notice a speed drop.

Many of the automatic connections are outlined in the How to stop Firefox from making automatic connections documentation, along with steps on how to disable them. While I certainly wouldn't recommend disabling all of them, there are probably some on the list that you can disable if you don't use it. Things like the Snippet feature, What's New, Add-on meta data, diagnostic data, etc.

But, before doing any of that, I'd probably recommend testing Firefox in Safe Mode first, to see if you notice a speed increase when you are in that mode. If you do, there may be an add-on installed on Firefox that's causing Firefox to slow down.

Hope this helps.

Also, out of curiosity, do you know what download speed you receive on your internet connection?

Connectivity monitor indicates 53.2 kbps, and fast.com in the best of cases reports 35-45, but usually zero due to these FF downloads, and perhaps some others. But anytime I see this unknown stuff start out of the blue, task manager always tells me its ff.exe. Booting is when this issue is at its worst.

Will look at what you have told me and see what happens. I have a third machine with same issues (for experimental PC-ology) so I won't be too concerned about it going down. Better than suffering this decay in speed, eventually eliminating my browsing.

Thanks for you helpful post, @Wesley Branton, and will get back once I have had a chance to check on this tomorrow. As a scientist, I know that even negative results can provide clues as to what is gong on!

No changes in safe mode. But the culprit at this point seems to be the preference "network.prefetch-next".

This was set to true, and changing it to false has made a big difference. Every time I do a fresh FF boot now, an immediate fast.com check always gives 35-45 kbps. This looks like a winner, but will keep all these notes and look for anything else.

Thanks again, and will update later if anything else is noted.

Was wondering if network.prefetch-next is going to get changed back to "true" on my next update FF update?

Typically speaking, the preferences in the about:config page aren't changed or reset during the update process, although there are some exceptions. That setting should stay the same for you though, even after an update.