To delete an email from the inbox in Firefox, you can follow these steps:

Open Firefox and log in to your email account. Go to the inbox folder where the email you want to delete is located. Locate the email you wish to delete. Hover your mouse cursor over the email to reveal the available actions. Look for the delete/trash icon, which is typically represented by a trash bin or a crossed-out garbage can. Click on the delete/trash icon to delete the email. Depending on your email client or provider, you may be prompted to confirm the deletion. If prompted, confirm that you want to delete the email. The email should now be moved to the trash or deleted items folder, depending on your email setup.

Hope this helps.

Two little suggestions:

Never put anything in Trash (alone) that you want to (even maybe) keep. Make copies and store them in Local Folders sub-folders.

Frequently empty the main Trash folder (the one above Local Folders). When your Trash folder is overflowing with recently deleted messages, the server might block deletions until you make some room in there.

Thanks for your comment. I believe I have a reasonable solution when this problem occasionally occurs but I was asking if anyone else thought this issue could be corrected by a Tbird update.

I think the issue is we really don't know what the issue is.

Deleting the files indicates there is an issue with that file. But what is the issue? Is it related to the file access in Thunderbird or something else external to Thunderbird even. I don't know.

I even wonder if the issue may be incorrect configuration after a change with the spectrum server. They have a long history of a buggy mail server product and it can cause issues in IMAP mail particularly as there is a synchronization occurring.

What is the folder specified in account settings to accept trash. For many servers it is "deleted", or even "Deleted Items"

Were there logical error on your hard disk that deleting the files removed? On windows you would use chkdsk to determine if logical error exists and fix it, but once you delete the file with the logic errors in it's allocation chkdsk will not see that error at least.

Has your antivirus at some point identified something it does not like in those files and blocked access? Quarantine sometimes changes the files name (not a problem) and others simply block access to it. (which would manifest as the problem you are seeing)

So if you want to help, when this issue occurs determining these sorts of things is necessary to begin requesting someone to fix the problem. Just knowing that occasionally you get this can't delete issue does not give anyone much of a start in tracking down where to look for the cause in a program with millions of lines of code. The more detail that can be provided with the initial bug report the more chance of someone being assigned to look into the issue.

Matt, Thanks for your detailed analysis about what it might take to understand the root cause of the problem. Unfortunately, my computer skills and knowledge are what I might call "age appropriate", that is, I'm 76 and a substantial portion of my life was spent in a pre-computer time period. Or to say it another way, I never learned most of the basic fundamentals of computers and internet functioning so I'm not going to be able to provide significant input in tracking down this problem. For me, just having a work around to fix it is good enough.