Maybe you could try what seems to have worked for this user :

https://answers.microsoft.com/en-us/outlook_com/forum/oemail-osend/outlookcom-beta-scroll-bar-keeps-jumping-back-to/da3dac79-b519-4887-b869-aff44774a50c

Clear the cache and cookies only from websites that cause problems.

Clear the Cache:

3-bar menu (or 'Tools') => Options => Privacy & Security => Cached Web Content :
"Clear Now"

Remove Cookies from sites causing problems:

3-bar menu (or 'Tools') => Options => Privacy & Security => under 'History' : choose 'Use custom settings for history' => "Show Cookies"

I am using FF 61.0.1 and did not see Cached Web Content or clear now. However I did go to Cookies and Site Data and clear data.

Also, I did choose 'Use custom settings for history' but did not see "Show Cookies"

I also used the CCleaner 544 pro trial on the firefox. So going under options to clear data and using ccleaner, both did not solve the problem yet.

lightalker said

I am using FF 61.0.1 and did not see Cached Web Content or clear now. However I did go to Cookies and Site Data and clear data. Also, I did choose 'Use custom settings for history' but did not see "Show Cookies"

I owe you an apology, cause I gave you outdated instructuctions (so sorry  !)

You already managed to find the proper place to clear both cookies and cache (well done  !) :

https://support.mozilla.org/en-US/kb/how-clear-firefox-cache

Note that clearing cookies will log you out of sites .....

I went to see if maybe somebody else had the same problem, and I came across this thread :

https://support.mozilla.org/en-US/questions/1125574

Could that be what's causing your problem  ?

The user on the Microsoft support forum couldn't pin it down, it seemed to have 'solved itself' - a bit vague ....