0.8 MB/s is a very low data rate for just about any modern storage device, never mind an HDD or SSD. The 99% you see is the total of all programs, so is there some other program that is hogging the rest? Downloaders, antivirus and malware come to mind first when the equation is Firefox + high disk use (high memory or CPU use are much more common).

As luck would have it I don't have my Windows 8.1 PC on hand to test what the typical usage is.

Whatever the case, if launching Firefox slows things down, try starting in Safe Mode: https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-using-safe-mode#w_how-to-start-firefox-in-safe-mode. If Safe Mode works much smoother, you may have overly heavy or overly many add-ons and need to do some house cleaning (note: Safe Mode does NOT disable Plug-ins!). You may also have a monstrous cache that needs clearing: https://support.mozilla.org/en-US/kb/how-clear-firefox-cache. Monstrous cache is compounded by keeping a high number of tabs open simultaneously, especially if they have rich media content and you don't use content blocking like AdBlock and NoScript.

Here is the general tips article on the subject, if you want to tweak things further: https://support.mozilla.org/en-US/kb/firefox-slow-how-make-it-faster. Usually managing cache and add-ons is enough.

Thank you, but I tried safe mode already and it does not change things. Also emptying the cache does not help.

Alright, in that case I would go after your profile next. Is it a new one or have you had it for a while?

An easy experiment to perform is to make a new profile, https://support.mozilla.org/en-US/kb/profile-manager-create-and-remove-firefox-profiles#w_starting-the-profile-manager, and see how things go from there. If the profile is very old and possibly dates back to another OS even (or has just simply become corrupt or taken on a bad setting), then it could well be the culprit.

I do remember some cases of Firefox being sluggish on W8 specifically, but those were cases of low power and/or external program interference like https://support.mozilla.org/en-US/questions/984666.

Thank you so much! A new profile seems to have done the trick!

You're welcome, glad we could pin down the culprit quickly!

Profiles can and do go bad over time, and sometimes an error creeps in, ruining everything - just have to remake it once in a while and you're golden.