There was recently a Firefox vulnerability exploited in the wild. It was patched very recently, so any build before that, ie August, would be vulnerable. Maybe it's that?
Discussion
Thanks, yeah, it's probably that. Just odd that they didn't patch it quickly like usual.
There was a problem with compiling some of the software in the Fennec F-Droid build chain from source. That's why it's taking F-Droid a while to ship an update;
https://forum.f-droid.org/t/fennec-vulnerability-recommended-to-uninstall/28826/39
In the meantime you can get more recent versions of mobile FF-based browsers by adding the DivestOS repo to your F-Droid client. But it requires uninstalling, then installing from that repo.
Thanks!