Hi, almost any solutions since Quantum 57 release does not work any longer. This seems to work well in Win10 : uninstall Firefox. Then Delete the Mozilla Firefox Folders in C:\Program Files , C:\Program Files(x86) & C:\ProgramData Then restart system. Then run Windows Disk Cleanup. (Note: This should be Pinned and run Weekly, If never done below expect 10's of gig's) Then run it again and click the button that says Cleanup System Files. Note: your Firefox Profile is saved. But you should make a back up before you do :

• https://support.mozilla.org/en-US/kb/back-and-restore-information-firefox-profiles
• https://support.mozilla.org/en-US/kb/export-firefox-bookmarks-to-backup-or-transfer

Reinstall with Current Release Firefox 59.0.2 with a Full Version Installer

• https://www.mozilla.org/firefox/all/

Please let us know if this solved your issue or if need further assistance.

I can only compare using Outlook Web App on Microsoft's infrastructure, not on a company server.

• [Consumer] https://outlook.live.com/mail/ => this was recently updated to new experience, and I don't notice any issues with the Backspace key.

• [Office365] https://outlook.office.com/owa/ => I don't notice any issues with the Backspace key.

In my test, I wrote some brief text, pressed Enter, click the numbered list button, created three items, pressed Enter twice to end the list, and typed some brief text. I then held down the Backspace key and removed everything.

Does that work for you? If that works, what are the circumstances where Backspace stops working for you?

Have you tested in Firefox's Safe Mode? In its Safe Mode, Firefox temporarily deactivates extensions, hardware acceleration, and some other advanced features to help you assess whether these are causing the problem.

If Firefox is not running: Hold down the Shift key when starting Firefox.

If Firefox is running: You can restart Firefox in Safe Mode using either:

• "3-bar" menu button > "?" Help > Restart with Add-ons Disabled
• Help menu > Restart with Add-ons Disabled

and OK the restart.

Both scenarios: A small dialog should appear. Click "Start in Safe Mode" (not Refresh).

Any improvement?

Also, your plugins list contains plugins that should not run in Firefox 52 and higher. Is there something custom about your Firefox build?

Thank you for you suggestions. I tried running Firefox in safe mode unfortunately that did not resolve the problem. Next, I performed the uninstall, folders deletion, then disk cleanup but that didn't fix it either. I may punt this over to Microsoft and see if they have any ideas.

This may very well be unrelated, but currently there is another bug related to keyboard shortcuts affecting Backspace key behavior in some cases. Can you check if that applies in some way by trying the workaround (change the permissions.default.shortcuts preference’s value)? If you think it does, try changing the permissions for overriding keyboard shortcuts for any OWA page as explained in the Firefox Page Info window support article.

Thank you for alerting me to this bug Tonnes. I tried the workaround but no change.

I just noticed Jscher2000 asking about the plug-ins. Yes, I do have old out dated plug-ins that I need to remove. I will eventually remember to do so. All my testing is done on a lab computer that just have Windows 10, Office 2016, and Firefox installed. No plug-ins are installed on this test computer.

If Microsoft doesn't offer a quick solution, it would be interesting to see what the HTML is that Firefox will not Backspace into, if that makes sense. If you select the entire message body (from the first letter through the signature, right-click > View Selection Source, you can see the structure of the message around the end of the list.

Note: this site is not good with pasted HTML, but if you want to share some sample message body HTML, you could use Pastebin or similar. Please scrub any sensitive information before posting.

https://pastebin.com/

You have read my mind. I am looking at the html right now.

Here is a screenshot.

It's a little hard to read... there also are some odd differences between the two. Was this composed purely in OWA? There seem to be some tags characteristic of Microsoft Word or Outlook on desktop; perhaps it's the stationery. Ignoring that for the moment:

Once you inserted the list item (Bullet point 1.), and then the fourth paragraph (Test Line 4), you can't Backspace through even the last text paragraph, or you can remove that but not the list?

Yes, it is all done in OWA. I typed in 'Test Line 1' and 'Test Line 2' copied the source to notepad then added 'bullet point 1' and 'test line 4' Once I added the bullet point, I cannot back space through any of the text. I can highlight and backspace to delete. I can also backspace to delete the line breaks.