.....

Office365 should work in your Firefox. Does Word online open if you go use this launch link:

https://www.office.com/launch/word

Or it could be safer to use your regular method of logging in to your Office365 account.

If that works, perhaps there is a problem with the way Dropbox is trying to send the document.

Also see :

https://www.dropboxforum.com/t5/Dropbox-files-folders/Recent-issue-Office-for-the-web-isn-t-supported-on-this-browser/m-p/402602

And :

https://www.dropboxforum.com/t5/Dropbox-installs-integrations/Message-Browser-not-supported/td-p/370301

jscher2000 said

Office365 should work in your Firefox. Does Word online open if you go use this launch link: https://www.office.com/launch/word Or it could be safer to use your regular method of logging in to your Office365 account. If that works, perhaps there is a problem with the way Dropbox is trying to send the document.

Yes, logging into Word Online straight from my Office365 account works. Only DropBox documents still won't open in Office365. Must be error on Dropbox's end. Thanks

McCoy said

Also see : https://www.dropboxforum.com/t5/Dropbox-files-folders/Recent-issue-Office-for-the-web-isn-t-supported-on-this-browser/m-p/402602 And : https://www.dropboxforum.com/t5/Dropbox-installs-integrations/Message-Browser-not-supported/td-p/370301

I tried the same - restarted my browser, cleared cache & cookies, disabled all extensions, but none of it worked. I'll ask DropBox Community.

I don't know if this has anything to do with it, but here its says :

"Can I add Dropbox as a Place in Office 365 Online?

Microsoft no longer supports Dropbox as a Place in Office 365 Online."

McCoy said

I don't know if this has anything to do with it, but here its says : "Can I add Dropbox as a Place in Office 365 Online? Microsoft no longer supports Dropbox as a Place in Office 365 Online."

Then I wonder why I still have the option to select Office365 online as my default editor for Word, Excel & PowerPoint documents.

I will wait what Dropbox Forum says

Hi Dunno, I tested with a Word document and I wasn't logged in to either an Office365 or consumer (live.com) account and Dropbox still was able to load the editor somehow.

https://support.mozilla.org/questions/1282214#answer-1298280

Perhaps there is a problem with cross-site communication permissions, if that has changed in Firefox 74 for some settings/options. I use the "Standard" level of Tracking Protection and tested in a regular (not private) window. I do not block third party cookies, if that is relevant.