Hello,
today, I updated via pihole -p to newest version.
Core v6.4.1 FTL v6.6 Web interface v6.5
Since then, I cannot use webgui in Firefox 115.34.0esr (64-Bit)
https://tricorder.pi-hole.net/anuS1dpT/
Of course, cache/cookies has been deleted.
What can I do?
That is the last Firefox release working on Windows 7 (which was EOL 2020) or older macOS 10.14 (EOL 2018) operating systems.
Currently, the Firefox major release is 149.
Not wishing to pre-empt developers comments, it seems possible that Firefox 115.34.0esr wouldn't support more modern HTML features as used by Pi-hole's web UI component.
If you are on a more current OS, you should consider upgrading to the latest Firefox release.
If you are on one of the outdated OSs, it could be worth considering to update to a current OS.
Use Pale Moon and you are all good : Pale Moon downloads
Works on a lot of older systems as long as the CPU is good enough features wise 
