''re :The 'Open With' option often, but not always, does not appear with email attachments. '' It only appears if you have not set up what Thunderbird is supposed to use to open them. If Thunderbird has already been set up to open eg: jpg files using Paint then it does not need to ask. Some people typically those using an iphone have a habit of creating images that are 'heic' file type and then redefining them has .jpg which is real nuisance. Maybe you have some jpg files that are really .heic files. HEIC files are the space-saving standard image format used by Apple across their devices. No one else uses it. It's an 'Apple' thing. Thunderbird can open .jpg and .jpeg files and they are set up to open using Paint. But those heic image files with .jpg extension are really .heic and therefore cannot be opened from within Thunderbird. You have to save to desktop first. I noticed in another question you were talking about image files but they were actually MS Word documents. https://support.mozilla.org/en-US/questions/1512414 I also noticed you said they opened once but then did not do it a second time - however, I could see the relevant icon on Task Bar showing the relevant program was already open and therefore probably containing the document you had previously opened. Selecting the relevant icon Task Bar would probably display the preopened document.