I can't say I've ever heard of this happening. When you downloaded Firefox originally, did you get it from the official Mozilla website?

I also assume this happens when trying to download any add-on from the Firefox Addon website?

First, I'd recommend trying to download an add-on while running in Firefox Safe Mode. This would rule out any issues caused by add-ons that may be interfering with the user agent (the information that tells websites what browser you are using).

If that doesn't help, the problem could be caused by a user.js file that you have in your Firefox profile. This is usually added by other programs or add-ons, but can sometimes cause issues with Firefox. Open your Firefox profile folder and rename the user.js file to user.js.old and then restart Firefox.

Hope this helps.

There are other things that need attention.

I notice that your System Details list shows that you have a user.js file in the profile folder to initialize prefs each time Firefox starts. The user.js file will only be present if you or other software has created this file and normally won't be present.

You can check its content with a text editor (right-click: "Open with"; do not double-click). The user.js file is read each time Firefox is started and initializes preferences to the value specified in this file, so preferences set via user.js can only be changed temporarily for the current session.

You can delete the user.js file if you didn't create this file yourself.

You can use the button on the "Help -> Troubleshooting Information" (about:support) page to go to the current Firefox profile folder or use the about:preferences page.

• Help -> Troubleshooting Information -> Profile Directory:
  Windows: Show Folder; Linux: Open Directory; Mac: Show in Finder
• http://kb.mozillazine.org/Profile_folder_-_Firefox

There is also an accessibility instantiator active:

• "UNKNOWN|C:\\Program Files (x86)\\Unchecky\\bin\\unchecky_bg.exe
• https://support.mozilla.org/en-US/kb/accessibility-services

Problem solved.

Wesley and cor-el, I tried your suggestions and they didn't resolve the issue. However I found a previous thread (https://support.mozilla.org/en-US/questions/1252955) and followed the suggestions which resolved the issue.

I found a user.js file in my profile, which I didn't create. I opened it and it was empty, so I deleted it. I also uninstalled the program that created the accessibility instantiator.

Thank you both for your assistance and suggestions.

Yes, you appear to have a weird user agent modified by Abelssoft software with Firefox 61.

• Mozilla/5.0 (Windows NT 10.0; rv:61.0) Gecko/20100101 Firefox/61.0 anonymized by Abelssoft 1998461859