The portable builds are fully portable, the profile directory is inside the browser folder. It does not touch appdata at all.
Changes on top of the previous 126.0 build:
Menus are no longer borked on 7 without DWM.
Scrollbar style and theme can be changed in preferences.
New about:config is accessible via about:confignew, classic about:config is still default.
Various CSS issues should be fixed now. (most notably there being no fog on 7 with system theme applied)
SERP telemetry is disabled.
Haven't tested on XP/Vista (extended kernel) nor have I stability tested.
Please let me know what bugs or issues you encounter with it.