Onion link
In that respect, Tor Browser 13.5 feels like a milestone: in addition to the dozens of bug fixes and minor improvements noted in the changelog below, this release features major changes to Android's connection experience in preparation for the future addition of Connection Assist, including full access to Settings before connecting and a new, permanent home for Tor logs.
For desktop, we're continuing our efforts to improve the user experience of Tor Browser's fingerprinting protections. Following the changes we introduced to new window sizes in Tor Browser 13.0 for Desktop, this release features welcome design changes to letterboxing, including new options to remember your last used window size and adjust the alignment of the letterbox in General Settings. Bridge users will also discover a myriad of improvements to bridge settings, including a complete redesign of bridge cards with improved sharing features, and a new section designed to help you find more bridges elsewhere. Lastly, the design of onion site errors has received a visual refresh aimed at making them consistent with the other kinds of Network Errors you can find in Tor Browser.
I know it's been a month, but if you are still interested. Mullvad is 95% Tor Browser without Tor (there are some other changes, but by and large it is). So besides the Tor network, there isn't a lot.
Librewolf I don't know a lot about. But I'm sure at least some of it's changes are just enabling about:config options that actually come from Tor Browser.