If you are downloading from https://www.thunderbird.net/ ... there are no known bad files there, so you should check that something else isn't interrupting the download.

If it's not from https://www.thunderbird.net/ then you need to complain to whomever hosts that website.

I don't understand what else could cause this. My error message when trying to download from the .net site is "C:\Users\PHILLI~1\AppData\Local\Temp\IWLra0qw.exe.part could not be saved, because the source file could not be read." Thanks for your reply, but unfortunately it leaves me nowhere.

Switch off VPN and firewall, try Chrome or latest MS Edge to download the file, or check for updates from Tbird Options-Help-About Tbird menu.

I don't use a VPN (to my knowledge). I do use Trend Micro Internet Security. I turned off Trend Micro and Windows firewall. Tried Chrome and Edge as well as Firefox. The same Firefox error message as above. The other error messages were non specific references to network. The file still wouldn't download. Any other suggestions are welcome, but at this point it looks like I need to start looking for another email client. I regret that because I like Thunderbird.

How do you get a Firefox message in chrome? A chrome error in edge I can understand as they are mostly the same except for the branding. With Edge being a rebranded chrome.

go to getthunderbird.com

Can you download from there? (a full reinstall will get the update.)

Here is an experiment that will indicate you turning off trend did not work, if it works trend was and still is the issue. http://ftp.mozilla.org/pub/thunderbird/releases/91.4.1/win64/en-US/ At the link you will see two files to download. They are both the 91.4.1 windows 64 bit Thunderbird version for the EN/US locale. (I did a lot of guessing) one is an EXE file the other an MSI. If you can download the MSI (windows installer file created for corporate use) but not the EXE (a self extracting ZIP file, something that Trend has been barking about for years) then I would be guessing the issue is Trend.

Note you can use the MSI file, but you have no control of where the software is installed. but it should be good for all windows versions since about 2003 as far as compatibility go and very few modify the default install anyway.

The getthunderbird.com link redirected to thunderbird.net with the same results as before. I tried downloading both the .exe and the .msi files you suggested (thank you). The msi wouldn't download with Firefox. The exe did download, after TrendMicro checked it for threats. (So the problem doesn't appear to be with TrendMicro.) When I tried to run the file I got a Windows error message saying the file was corrupted and couldn't be run. Any other thoughts are still welcome and thank you again for your time and trouble.

Check your PC hard drive for filesystem errors in its Properties-Tools, do disk cleanup with Clean System Files checked, or try to download to a USB thumb instead. As an option, do full system & Registry cleanup, for example with free Wise Care 365 (I use it once a week) or similar package.

disable trend micro from scanning downloads and try again. Both of your error will have a source there I would think.

Any file scan events would still show up in TM threat block log accessible through its GUI.

Matt, thanks very much. I turned off Trend Micro and downloaded .exe file with Edge and it worked. new T-bird is now installed and working (apparently). No error messages encountered.