Some websites require to obtain the time-zone from the client browser in order to correctly display some information.
This is not always critical but in case you are using some web hosted e-mail or calendar account this is important.
One day I noticed that suddenly all my tutanota.com mail calendar events were completely off by 2 hours.
By looking around I found a similar issue reported on the Mozilla support https://support.mozilla.org/en-US/questions/1247616#answer-1191679
After reading the solution
in case you’ve set privacy.resistFingerprinting to true in about:config, firefox will report utc time to websites…
obviously I remembered that some days before I actually activated explicitly “privacy.resistFingerprinting” feature from Firefox: https://support.mozilla.org/en-US/kb/firefox-protection-against-fingerprinting.
Because the time-zone is one of the most used variables that websites try to link to your unique ID, by activating this feature, the Firefox browser just gives some default value to try to trick the trackers.
The workaround is simple. Next to the “privacy.resistFingerprinting=true” there is another directive that must be set under about:config in the Firefox Browser:
“privacy.resistFingerprinting.exemptedDomains = *tutanota.com,*voina.org”
The above will return to the websites from the selected list the correct time-zone.