It works fine here, for plain text and html messages. Setting mailnews.wraplength to 0 (zero) in Config. editor makes plain text messages wrap to the window width. HTML wraps to the width automatically.

Does the same thing happen in safe/troubleshoot mode (hold Shift when launching TB)?

I created a new profile for 128, as I do with every major release. Are you using a profile from an older version? Creating a new profile with 128 is recommended over trying to downgrade to 115. Help/Troubleshooting Info, about:profiles, to create and manage profiles.

Thank you, sfhowes, for your reply.

I've tried starting in safe mode but the problem persists even then. I've tried clearing the cache, no difference. The odd thing is that the problem is affecting two different computers, admittedly in the same house but not networked together. We have a slightly dodgy internet connection over 4G - could that have anything to do with it? We're also in France, is this in any way relevant?

Setting the mailnews.wraplength to 0 made no difference to either plain text or HTML.

I've never created or changed a profile; the one I seem to be using is called default-release and, looking at the dates in the folder, it is a profile that is indeed several years old. Could you please tell me why creating a new profile will change how the text wraps? Should I be changing settings (and if so, which ones?) in the new profile? Sorry, but this is uncharted territory for me and I don't know what I'm doing here.

I can't say exactly why it's not working for you, but I don't think the connection type or locale matters. But I'm pretty sure it will work in a new profile, and it will be much quicker than trying to debug your current profile, even if you haven't dealt with profiles before. In theory, updating TB with the previous profile should be seamless, but in practice there are often old files and settings that hamper the migration. An example.

Open the Help/Troubleshooting Info tab, click about:profiles, create a new profile and give it a name, then add an account and see if wrapping works properly. Change the mailnews.wraplength preference in the new profile for plain text wrapping.

The old profile is unaffected and can be opened as before or from about:profiles. If the new profile works, local mail, address books etc. can be copied from the old to the new. IMAP accounts, network calendars and contacts will sync with the mail server after being added to the new profile.

Thank you so much, sfhowes, for your help. I have now created a new profile, set the wraplength to 0 and unfortunately it has made no difference.

This is all very peculiar... where do I go from here?

There is no shortage of posts about wrapping issues, as a web search shows. But I can't find any that point to a sure solution in this case. An earlier topic suggests adding a preference in Config. editor. Open the editor and paste mail.compose.wrap_to_window_width into the search box. Since the pref. isn't there by default, click the + icon to add it as a Boolean with the value true. This pref. existed in old versions, but was removed due to it causing other problems. I would be (pleasantly) surprised if it helps.

https://support.mozilla.org/en-US/questions/1161310

I also wonder if you have an external app like CCleaner that resets preferences in apps like TB to override manual settings.

That was a brilliant thought, sfhowes; I had high hopes for this one... sadly, it did not work. And no, I don't have CCleaner or anything like that. Since I moved to Windows 10 I've let Windows deal with cleaning the registry.

I'm starting to think that I'm going to have to to uninstall TB on both computers and re-install from scratch, which is a bit of a nuisance. Unless you have any further ideas?

Thank you so much for keeping on trying to help me with this problem!