Firefox has had known issues with various email service providers at some point and sometimes Edge is what I would recommend when using Microsoft services or emails services. So what do you mean "other devices" this is not clear what devices and O/S those devices are here?

Firefox has had known issues with various email service providers at some point and sometimes Edge is what I would recommend when using Microsoft services or emails services. So what do you mean "other devices" this is not clear what devices and O/S those devices are here?

I accessed Outlook just fine on my cellphone and another computer, which made me think it could have been a hardware issue with my PC, but I'm almost certain it's a browser issue.

In the meantime I will just use Edge for opening emails on Outlook.

Those devices where they using Firefox or their specific browser other then Firefox? Another example take Ebay or Amazon if opened in Firefox there are missing text or pictures but when using Chrome all the text and pictures shows up. So those could be the nature of the Browser programming that it can't use all the sites script or programming to see what is on their site as well. So this is why is have Edge for MS services and Chrome for Ebay and Amazon sites as such. I just find Firefox is good for some but for some reason big sites they don't work well with.

If you use extensions ("3-bar" menu button or Tools -> Add-ons -> Extensions) that can block content (Adblock Plus, NoScript, DuckDuckGo PE, Disconnect, Ghostery, Privacy Badger, uBlock Origin), always make sure such extensions do not block content.

• make sure your extensions and filters are updated to the latest version
• https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems

You can try these steps in case of issues with webpages:

You can reload webpage(s) and bypass the cache to refresh possibly outdated or corrupted files.

• hold down the Shift key and left-click the Reload button
• press "Ctrl + F5" or press "Ctrl + Shift + R" (Windows,Linux)
• press "Command + Shift + R" (Mac)

Clear the Cache and remove the Cookies for websites that cause problems via the "3-bar" Firefox menu button (Settings), click the padlock icon (Clear cookies and site data) for the domain in the selected tab.

"Remove the Cookies" for websites that cause problems:

• Settings -> Privacy & Security
  Cookies and Site Data: "Manage Data"

"Clear the Cache":

• Settings -> Privacy & Security
  Cookies and Site Data -> Clear Data -> [X] Cached Web Content -> Clear

• https://support.mozilla.org/en-US/kb/clear-cookies-and-site-data-firefox
• https://support.mozilla.org/en-US/kb/how-clear-firefox-cache

Start Firefox in Troubleshoot Mode to check if one of the extensions ("3-bar" menu button or Tools -> Add-ons -> Extensions) or if hardware acceleration or if userChrome.css/userContent.css is causing the problem.

• switch to the Default System theme: "3-bar" menu button or Tools -> Add-ons -> Themes
• do NOT click the "Refresh Firefox" button on the Troubleshoot Mode start window

• https://support.mozilla.org/en-US/kb/diagnose-firefox-issues-using-troubleshoot-mode
• https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems

You can check for issues with Enhanced Tracking Protection. Firefox shows a purple shield instead of a gray shield at the left end of the location/address bar in case Enhanced Tracking Protection is blocking content.

• click the shield icon for more detail and possibly disable the protection

You can check the Web Console for relevant-looking messages like blocked content.

• https://support.mozilla.org/en-US/kb/enhanced-tracking-protection-firefox-desktop
• https://support.mozilla.org/en-US/kb/total-cookie-protection-and-website-breakage-faq
• https://support.mozilla.org/en-US/kb/trackers-and-scripts-firefox-blocks-enhanced-track

Troubleshoot mode couldn't help me find the problem. I just uninstalled and reinstalled the app, keeping all my settings the same, and the problem was fixed, so all is good now. Still not sure what it was.