Quoting, pasting and setting markup broken recently

I’m having a lot of problem quoting material recently. Sometimes marking the original and clicking quote works, but often it does nothing. Sometimes a copy and paste works, other times it fails to paste, but if I paste into gedit, and re-copy it does paste. Sometimes the </> button does nothing, but you can still manually enter back ticks.

The quote of the authorisation header in Can't register from external LAN - #8 by david551 had to be done by staging a copy through gedit, and manually entering backticks.

I’m using Firefox 78.14.Oesr(64-bit) on a fully updated Debian 10 (Buster) amd-64.