So is it internet, is it the browser slow to respond, please provide detail to your statement and expand on it. There is no information to work with.

Start Firefox in Safe Mode {web link} by holding down the <Shift> (Mac=Options) key, and then starting Firefox.

A small dialog should appear. Click Start In Safe Mode (not Refresh). Is the problem still there?

https://support.mozilla.org/kb/Firefox+is+already+running+but+is+not+responding

https://support.mozilla.org/en-US/kb/firefox-slow-how-make-it-faster

https://support.mozilla.org/en-US/kb/firefox-uses-too-many-cpu-resources-how-fix

https://support.mozilla.org/en-US/kb/firefox-uses-too-much-memory-ram

https://support.mozilla.org/en-US/kb/firefox-hangs-or-not-responding

Can move to this if not Extensions. As well provide this please : Type about:support in the address bar and press Enter. Under the main banner, press the button; Copy Text To Clipboard.. Now in the Reply Box on the forum page, do a right-click in the box and select Paste.

This will show us your system details. No Personal Information Is Collected.

You can try this while also providing the above information : uninstall Firefox. Then Delete the Mozilla Firefox Folders in C:\Program Files and C:\Program Files(x86) Then restart system. Then run Windows Disk Cleanup. (Note: This should be Pinned and run Weekly, If never done below expect 10's of gig's) Then run it again and click the button that says Cleanup System Files. Note: your Firefox Profile is saved. But you should make a back up before you do : https://support.mozilla.org/en-US/kb/back-and-restore-information-firefox-profiles

https://support.mozilla.org/en-US/kb/export-firefox-bookmarks-to-backup-or-transfer

Reinstall with Current Release Firefox 57.0.2 with a Full Version Installer https://www.mozilla.org/firefox/all/

Please let us know if this solved your issue or if need further assistance.

The fact that my other browser (Edge) performs just as well as it did before the Firefox upgrade would indicate that the problem is not "internet," whatever you mean by that. My Wi-Fi has been consistent, and whether I just have Firefox open or both Firefox and Edge open at the same time, Firefox is amazingly slow. If both browsers are open, Edge works at normal speed, even though it has its own bugs in other areas. Since I upgraded Internet Explorer to Edge a few months ago, I have had some bug issues with it, but never a problem with slow speed. Firefox is consistently slow on every website I have tried (at least 10, waiting for this browser wastes a lot of time), while Edge consistently performs at normal speeds. It doesn't matter whether I have other folders open, other browsers open, or other apps open, Firefox is slower than any other browser I have ever used. Before the latest upgrade, which I installed about a month ago, Firefox worked just as well as the other browser I have; now it is crap.

 I have already gone to your help forum and tried the solutions listed there; the links you've provided sound like the things I've already attempted. I tried starting in Safe Mode and nothing was different, and most of your other instructions here are unintelligible. I do run Disk Cleanup on a weekly schedule, along with Defrag, but I ran Disk Cleanup again 2 days ago just to be sure. I have Windows Defender antivirus running continuously, but just to be sure, I ran a full scan (several hours' time, just as usual) on the computer; no malware was found. Even though I just recently downloaded the Firefox upgrade, I went ahead and downloaded the whole browser again today from the Firefox website; again, nothing improved. I'm assuming that Firefox supplies a current version on its own website.
 My laptop, the only computer I use aside from my smartphone, still works as well as it ever did (it's about a year old) aside from this problem. Our Wi-Fi is paid up and working fine for my other browser and for the desktop computer used by others in my household. My phone's Google Chrome browser works fine, but it usually uses the Verizon network for internet, not Wi-Fi; its network is slower than Wi-Fi, but still much faster than Firefox on this laptop. I haven't noticed any other recent changes on the laptop, desktop, or any smartphones.
 In terms of speed, I would estimate that a typical navigation action on the new Firefox takes 20+ times as long as it does on Edge or Google Chrome. This is an estimate of course, but it seems consistently this slow no matter what I am attempting to do.
 I keep my laptop plugged in all the time and take care to keep it from overheating. We haven't had any power outages in the area for months (and that one early this fall was a rare one) nor have we had any power surges; anyway, the power strips my computers are plugged into have surge protectors. We have not dropped or damaged either of these computers since their purchase, nor have we spilled anything on them; the aquarium is in a different room from any computer equipment. My anti-seizure medication is working well and I haven't had any issues with my sense of time. I have no illegally obtained equipment on either computer, and to the best of my knowledge I have no pirated files. As far as I know I am not subject to any law enforcement investigations, ex-wife revenge plots (I've never been married), or terrorist cyber-attacks, nor am I aware of any alien implants in my body. I haven't left the USA in at least 10 years.
 I hope this is enough information to work with. I am totally not being sarcastic.

Well, when you decide to reply to the suggestions I posted and confirmed you have proceeded with instructions and tried them and what the results were. Will help you some more.

The above post has nothing to do what so ever with helping you and the information provided and the results of your actions doing this to Firefox and reporting back about how these things helped or did not help.