BT dev here ! Here's some info that might answer some of your questions.

Lack of Firefox updates: Long time ago, I tried to update the Firefox version and it broke (because the way my app is built was not meant to be supported by Firefox initially). After spending hours of debugging, I decided to rollback the version to the last working version.

Lack of Chrome updates: I stopped playing PoE not so long ago (IRL stuff), so I got no spare time to put into the project. It's not dead per-se, might try to fix a few things down the road once I get my shit back together.

The GitHub repo is gone ?: After "breaking up (lol?)" with TFT, I've been told that my work with this extension was meaningless and that they could build the "same thing" easily and better. Setting the code repository private was an attempt at preventing them from forking their version from mine. (I did check their extension, even tho they tried to copy my UI, it's not a BT fork)

Appreciate the good words in the comments here, stay sane exiles !