After several months away from SL, I logged in last night (after updating my SL and Firestorm viewers to the current release versions). The problem appears to be limited to the SL viewer.
Prim hover (floating) text, i.e. created by llSetText(), is not appearing when I'm using the SL viewer (it wasn't appearing when I initially logged into the Firestorm viewer, but restoring old Firestorm settings fixed it for Firestorm).
Bizarrely, this type of floating text can be seen on attached HUDs, but the HUD prims can't be seen-yet the HUD seems to function properly.
The problem is independent of avatar (I saw it on the two avatars I tried).
I tried uninstalling and doing a totally clean install of the SL viewer; the problem still exists.
Is there some obscure setting I'm missing? I've looked through pretty much everything I could think of (it's annoying enough to find settings that used to be on the main menus now buried in the Debug Settings menu).
SL viewer: Second Life Release 6.0.1.522263 (64bit)
Firestorm viewer: Firestorm 5.1.7 (55786) Jul 13 2018 22:34:51 (64bit) (Firestorm-Releasex64) with OpenSimulator support
Windows 7 64 bit
First image in SL viewer, second in Firestorm