The way to keep it in homebrew is to either start signing librewolf builds or provide an alternative tap from Librewolf's repo. One of the catalysts for the deprecation is that installing unsigned apps seems to only work in Intel-based macs and Apple has decided macOS 26 will be the last release that will support this architecture.

I don't know all of the brew lingo, but I think using the cask option/command uses the DMG. That's what I use when I saw that message a while ago

The commit says something about something being “unsigned.” I suspect that means the LibreWolf artifact is unsigned, so they can’t verify its integrity.
https://github.com/Homebrew/homebrew-cask/pull/221666/files#diff-a2f494476a9a3132aa0da63b28d9fa05eb7180e78c32148864c7cdc214df33b9R22

The same thing just happened to Chromium:
https://github.com/Homebrew/homebrew-cask/pull/221570/files#diff-7073d00a063da1cbaa54537a0299fc88c89e55c6a7a0e0de2c34e82752d90e3aR13

Edit to add: Perhaps this was done in error, and perhaps it’s related to this issue: Autobump restrictions make opening PRs with bump-cask-pr overly complicated