It sounds like the installation didn't complete correctly. What I usually recommend in that situation is:

Clean Re-install

This takes about 5 minutes. It's not about your settings, it's about making sure the program files are clean (no inconsistent or alien code files). As described below, this process does not disturb your existing settings. It's not essential to uninstall Firefox, but you can if you like, saying No to any request about removing personal data.

If you want to make a backup first just in case, see: Back up and restore information in Firefox profiles

(A) Download a fresh installer for Firefox to a convenient location:

https://www.mozilla.org/firefox/all/

(B) Exit out of Firefox (if applicable).

If you have Microsoft Office installed locally, please change your system default browser to Internet Explorer temporarily before the next step.

(C) Using File Explorer (hold down the Windows key and press E to launch it), right-click > rename the program folder as follows (you might have one or both):

C:\Program Files (x86)\Mozilla Firefox =to=> C:\Program Files (x86)\OldFirefox

C:\Program Files\Mozilla Firefox =to=> C:\Program Files\OldFirefox

(D) Run the installer you downloaded in step (A). It should automatically connect to your existing settings.

Any improvement at your next startup?

Thank you. I didn't need to do this. I rebooted my computer and was able to access Firefox although it is now really, really slow in loading pages.

It is possible that your firewall or other security software blocks or restricts Firefox without informing you, possibly after detecting changes (update) to the Firefox application. Remove all rules for Firefox from the permissions list in the firewall and let your firewall ask again for permission to get full, unrestricted, access for Firefox and the plugin-container process and the updater process.

See:

• https://support.mozilla.org/en-US/kb/server-not-found-connection-problem
• https://support.mozilla.org/en-US/kb/Firewalls
• https://support.mozilla.org/en-US/kb/fix-problems-connecting-websites-after-updating

Thank you. I didn't need to do this. I rebooted my computer and was able to access Firefox although it is now really, really slow in loading pages.

Does Firefox seem to be using an unusual amount of CPU? You can call up the Windows Task Manager using Ctrl+Shift+Esc. On Windows 7, check the Processes tab to see the CPU used by different firefox.exe processes. On Windows 10, check the Details tab to see the CPU used by different firefox.exe processes.

If it seems excessive relative to the sites you are visiting, check for malware: Troubleshoot Firefox issues caused by malware.