Can you empty the cache, delete the cookies?
Start firefox in save mode.
In my case nothing helped only after some weeks everything was normal again.
May be your pc want to tickle you - if you ignore it it gets bored after some time..
Yes, I've tried deleting the cookies and cache.
I don't know how to start Firefox in save mode.
I have put a ticket in to the service desk to see if they have any suggestions. Hopefully it is something that they have come across before.
As the problem is on your side, the service desk can't help you.
To start FF in save mode follow my link: firefox -safe-mode
You may try as well 'in escalating order':
1) in FF create a new profile.
2) reinstall FF
3) update your Windows
4) start as a new Windows user
5) re-install Windows
6) buy a new pc
Or wait and see - I guess it'lll come back by itself
Service desk came back with a solution.
I needed to adjust the zoom on the page. In my case, I had to set it to 90%.
They were aware of the problem and said that they will fix it soon.