Providing an example and link would help us help you.

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 "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

Extensions may not have set to run in PB mode windows.

• https://support.mozilla.org/en-US/kb/extensions-private-browsing

A major difference between a regular window and a private window is that PB windows keep all data in memory and do not use the disk cache, but use a separate cookie jar kept in memory, cookies from regular windows stored in cookies.sqlite in the profile folder can't be accessed. All PB mode windows share the same PB mode cookies jar, you need to close all PB windows or close and restart Firefox to start with a fresh PB cookies jar.

Troubleshooting at least allows Firefox to load quickly, but the issue of not being able to sign in still persists. If I create a new profile, how can I import my passwords?

You can copy certain files with Firefox closed from one profile folder to another profile folder to transfer your personal data. Note that best is to avoid restoring a full profile folder and only restore important files that are safe to restore/transfer.

• https://support.mozilla.org/en-US/kb/recovering-important-data-from-an-old-profile

• bookmarks and history: places.sqlite
• favicons: favicons.sqlite
• bookmark backups: compressed .jsonlz4 JSON backups in the bookmarkbackups folder
• cookies.sqlite for the Cookies
• formhistory.sqlite for saved autocomplete Form Data
• logins.json (encrypted logins) and key4.db (encryption key/primary password) for logins saved in the Password Manager
• cert9.db for certificates stored in the Certificate Manager
• persdict.dat for words added to the spell checker dictionary
• permissions.sqlite for Permissions and possibly content-prefs.sqlite for other website specific data (Site Preferences)
• sessionstore.jsonlz4 for open tabs and pinned tabs (see also the sessionstore-backups folder)