Disclaimer: This is not meant to attack the Tor Browser devs, since they are in a pretty unique situation which forces their hand for many of these regressions.
The biggest regression is using ESR and all the baggage that comes with it. Partial security backports, fewer eyes for security issues and missing the churn that regular releases get. I’d argue the new code being added is better, largely thanks to the AI usage Mozilla has integrated.
Because of fingerprinting concerns, there’s no content‑blocking at all (except in the Tails version, which ships with uBO), so you have to “raw‑dog” the internet.
CRLite is also disabled, forcing reliance on OCSP for certificate revocation, a system that’s being phased out.
On Linux you’re forced to run it under X11 or XWayland, which defeats the improvements Wayland brings over X11.
You can’t selectively disable settings to improve security without breaking the fingerprint‑protection it provides, so you have to switch modes instead, introducing its own set of caveats.
This is all on top of the security issues Firefox by itself already has.
There might be some more stuff I am missing, but I don’t wanna spend too much time on this.