I just recently noticed this on my work laptop first, but then saw the same thing happening on the PC I'm remoted into via MSTSC. Once I restarted my laptop, the copy/paste started working again on both machines. My work laptop has various enterprise security software on it, so my best guess is that something wonky went on with whichever service monitors the clipboard access and wasn't allowing clipboard access to FF for some reason.
I cant say this is the same for everyone, but if you are using a work laptop that has some security monitoring software on it (CarbonBlack, ThreatLocker, Thycotic, BeyondTrust, etc) and rebooting doesnt help, hit your IT security team up to see if they have some policy about FF in particular.
EDIT: Ok wait I take some of that back (maybe) - Everything was working after I restarted, but a couple minutes after remoting into my other desktop, copy/paste stopped working in FF. Once I closed my RDP session, it started working again. Re-connected to RDP, and it stopped working.
It was working for weeks up to this point so i'm not sure why it does this. I like blaming security software for everything so I'm still saying its related to that. Or something related to the MSTSC Clipboard access
