Thank you very much for the add-on. Now on to the issues:

1) With certain Firefox themes, when add-on is disabled it's pretty much invisible on the panel. Please fix. A sample theme https://addons.mozilla.org/en-US/firefox/addon/linen-light/

2) Please if possible implement a list of exceptions. To ease the pain please consider subdomains from the get go, i.e. when I whitelist abc.com, domain.abc.com gets whitelisted automatically

3) https://browser.geekbench.com/v5/cpu breaks with your add-on enabled and shows "Invalid Date". Please fix.

4) Breaks far too many websites for comfort. Looks like instead of changing the web browser TZ, it actually modifies certain JS code and it does so incorrectly in many cases. I have to uninstall it. Too much hassle.

use this feature you will not be able to pass hcaptcha.com

Mostly works; does not account for daylight savings time, and preferences page is awkward. I'd suggest a simple list of 10-12 zones to cover the majority of users, and an advanced list (in alphabetic order) for the rest.
Edit, after a few days of use: breaks a number of sites, including flight booking at westjet.com (calendars do not work at all) and managing my Mikrotik router (all dates are incorrect by a day.)