The question is so general no one reading is going to know what site or what is happening here. So you need to provide screenshot of the error or URL to verify what is happening during the login problems.

Do you use a bookmark or are you starting with the main (home) page of this website? If you use a bookmark to access a specific page, instead navigate to this page starting with the main page or with the sign in page in case there is a problem with this bookmark. ---- 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can try these steps in case of issues with webpages: You can reload webpage(s) and bypass the cache to refresh possibly outdated or corrupted files.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<br>Cookies and Site Data: "Manage Data" "Clear the Cache": *Settings -> Privacy & Security<br>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 <b>Troubleshoot Mode</b> to check if one of the extensions ("3-bar" menu button or Tools -> Add-ons -> Extensions) or if <b>hardware acceleration</b> or if <b>userChrome.css/userContent.css</b> is causing the problem. *switch to the Default System theme: "3-bar" menu button or Tools -> Add-ons -> Themes *do NOT click "Refresh Firefox" in 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 and Total Cookie 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 about 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 ---- Reboot the computer into Windows Safe Mode with network support to see if that has effect. *[https://www.bleepingcomputer.com/tutorials/how-to-start-windows-in-safe-mode/] *[https://www.digitalcitizen.life/4-ways-boot-safe-mode-windows-10/]