On suitable PCs you should use the 64-bit versions of Firefox and Thunderbird. If yours is still running on 32 bit, the conversion is not a problem.
Since the two Mozilla programs have been around for so long, there are installations on many PCs that have already “hung up” quite well. So it could be that you still have the 32-bit version installed. When in doubt, people used to use the 32-bit version. On suitable PCs, however, the 64-bit versions are now preferable - at least mostly.
In addition, 64-bit applications can make better use of the main memory. Last but not least, sooner or later Microsoft could reduce support for 32-bit applications.
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Use 64-bit versions, ...
- If your Windows is installed in the 64-bit variant. Check this by typing in the File Explorer in the left column with right up This PC click and the Characteristics to open. The system info should say: "64-bit operating system, x64-based processor".
- If your PC has at least 4 GB of RAM. You can find this information in the same place, under «Work memory» or «Installed RAM».
- If you do not use 32-bit add-ons in Firefox or Thunderbird. Add-ons are additionally installable extensions. Which ones you have can be found in Thunderbird and Firefox via Hamburger Menu/Addons and Themes out of here. You probably won't find any extensions already installed in Thunderbird. Most common, current Firefox add-ons should also be usable in the 64-bit version. I didn't miss any after I switched - and that was a year or two ago.
This Windows 11 runs on the 64-bit architecture and has enough RAM easily
You may already have the 64-bit versions of Thunderbird and Firefox. Check it out by doing trans Hamburger menu top right to Help/About Firefox or. About Thunderbird walk.
The information window under the name says whether it is a 32- or 64-bit installation.
I intentionally installed the 32-bit version of this Thunderbird for demo purposes