This may be a glitch on Facebook's side in the process of switching to the new desktop layout. I.e., the old layout is adversely affected by some feature or change in the new layout. I've seen a few of these issues come and go during the last weeks. Right now they're testing and changing a lot in the new layout. There is pretty much nothing we can do about it. Everything is changing too quickly and the old layout will be obsolete in a few months, anyway, so it's really not worth even thinking about fixing it.
Officially, Facebook does not support TenFourFox with its new desktop layout even though the new one works better and quicker for our browser than the old one. A prerequisite to seeing it is changing the useragent to Firefox 52ESR or higher (Preferences>TenFourFox). It's not offered to all users, yet. As usual, Facebook rolls out big changes over a long period of time. I cannot say yet what will happen to TFF users when they switch off the old layout completely and force all users to the new one.
I can confirm the bug you describe in the old layout. The new layout is not affected by this.