You can try these steps in case of issues with web pages:

You can reload web page(s) and bypass the cache to refresh possibly outdated or corrupted files.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 cookies from websites that cause problems.

"Clear the Cache":

• Firefox/Tools > Options > Advanced > Network > Cached Web Content: "Clear Now"

"Remove Cookies" from websites that cause problems:

• Firefox/Tools > Options > Privacy > "Use custom settings for history" > Cookies: "Show Cookies"

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

• Switch to the DEFAULT theme: "3-bar" menu button/Tools > Add-ons > Appearance
• Do NOT click the Reset button on the Safe Mode start window

• https://support.mozilla.org/kb/Safe+Mode
• https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes

The only method that worked was 'safe mode.' It made it look the way it's supposed to, but disables all my addons, so it's not convenient. Is there any other solution? I notice that the web pages are opening in httpS mode. Could that be it?

I just noticed though that whenever I pull up the Tumblr website, it shows a green lock next to the address bar. It doesn't do this with any of the other websites I open. Could that be a reason why this is happening?

Firefox 42+ has a Tracking Protection feature that is enabled by default in Private Browsing mode. When you are in private browsing mode then Firefox can show a shield icon at the left end of the location/address bar that some content is affected.

You can disable this feature in "Options/Preferences > Privacy" or via the about:privatebrowsing page that get when you open a New Private Window.

• Options/Preferences > Privacy: "Use Tracking Protection in Private Windows"

• https://support.mozilla.org/kb/tracking-protection-firefox
• https://support.mozilla.org/kb/control-center-site-privacy-and-security-firefox

I have this problem too. I am running under Windows 10. The view changed from desktop to mobile on one site 5 days ago. Today, it changed on another site.