Maybe try an uninstall and reinstall.

Before you uninstall in Settings -> Privacy & Security -> Cookies and Site Data, clear all site data and cookies.

That’s all I got. Best of luck!

So its likely not your ISP or other site issues. Looks like in another comment you said it could be an add-on. Like debugging skyrim disable all addons and enable then one at a time till you find the problem. If you do find the problem you might want to check the addons git and report a bug if not already found.

Try a useragent switcher and spoof a different browser. See if its the sites you visit are throttling you for using FF. Worth a test.