Hello doom_br. As far as I know, the browser syncs with the Windows date and time. Please check first if the date and time of the Windows is correct. I understand that you use “applications, softwares, web services”, that means it can be from those apps that they did not update to the correct timezone.
You can try as a workaround this extension Change Timezone (Time Shift) – Get this Extension for 🦊 Firefox (en-US) that can manually change your timezone and you can set by hand +/- hours. To do that, install the add-on, right click on it → Manage Extension → Options → choose from the list the timezone you want. If you don’t find the timezone you want in the list, you can set it manually by filling in the name of the timezone you want to appear and then clicking the “+/-” buttons. I changes by 30 minutes. See the images attached.
If that add-on doesn’t work, you can try another workaround that is a little bit harder to do.
Go to user.js file which you find it by searching in Windows explorer “%appdata%” → Comodo → IceDragon → Profiles → (open the folder) → scroll down to the “prefs” file and right click on it
and click open with Notepad++ (you have to install Notepad++ app to open it). After it opens, scroll down to the last line of code and insert “user_pref(“mailnews.display.original_date”, true);”, without the quotes and save the file. This line of code makes the browser to not convert the timezone that the mail app has to the one that Windows has. More info on this link Time and time zone settings - MozillaZine Knowledge Base.