Despite being a new, reformed Microsoft, it seems the Seattle company is still able to interfere with a 3rd party browser. Mozilla has had to halt the roll-out of their new Firefox 66 browser due to an issue with PowerPoint Online.
Reportedly the updated browser is unable to save text when typed into the online presentation software.
Mozilla is currently testing a fix in their Firefox Nightly builds, but until then the main roll-out has stopped.
If you have already installed the build and need to use PowerPoint Online on Firefox, you can address the issue by changing the preference dom.keyboardevent.keypress.hack.use_legacy_keycode_and_charcode value to powerpoint.officeapps.live.com. Reloading the page should implement the fix.
Mozilla may use their Normandy remote preference update system to push out the fix to everyone, once the fix is properly tested.