Menu -> Help -> Troubleshoot Mode... Firefox will restart and a Troubleshoot Mode options window will appear. Do NOT select Refresh.

With Firefox closed, first make sure Firefox is not running in the background, then, hold down the Shift key while launching Firefox. (If the Profile Manager opens, select a profile and then hold down the Shift key again while clicking the "Start Firefox" button.)

Troubleshoot Mode is a debugging startup mode available where all added extensions are disabled, the default theme is used, and default localstore settings (toolbar settings and controls) are used. Troubleshoot Mode also disables hardware acceleration, the Just-in-time (JIT) JavaScript compiler, and any changes made via userContent.css and userChrome.css. Any changes made to preference settings remain in effect in Troubleshoot Mode, however, and all available plugins are used.

To summarize:

Troubleshoot Mode temporarily affects the following: All extensions are disabled. The default theme is used, without a persona. The Just-in-time (JIT) JavaScript compiler is disabled. The userChrome.css and userContent.css files are ignored. The default toolbar layout is used. Hardware acceleration is disabled.

Safe Mode has no effect on the following: The status of plugins is not affected. Custom preferences are not affected.

Diagnose Firefox issues using Troubleshoot Mode https://support.mozilla.org/en-US/kb/diagnose-firefox-issues-using-troubleshoot-mode

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

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 about blocked content.

• https://support.mozilla.org/en-US/kb/enhanced-tracking-protection-firefox-desktop
• https://support.mozilla.org/en-US/kb/smartblock-enhanced-tracking-protection
• https://support.mozilla.org/en-US/kb/trackers-and-scripts-firefox-blocks-enhanced-track

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).

"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 userChrome.css is causing the problem.

• switch to the DEFAULT 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

• https://support.mozilla.org/en-US/kb/enhanced-tracking-protection-firefox-desktop
• https://support.mozilla.org/en-US/kb/smartblock-enhanced-tracking-protection
• https://support.mozilla.org/en-US/kb/trackers-and-scripts-firefox-blocks-enhanced-track