Are you by any chance using Avast to secure your computer?

The reason I ask is because Avast has a called HTTPS Scanning that can significantly slow the TLS handshake process or cause it to fail sometimes, most noticeably when you first open Firefox in many cases.

This setting can be found in the Avast settings if you selection Protection and then Core Shields in the Web Shield section. It's called Enable HTTPS Scanning. Disabling that and then restarting your computer can possibly fix the issue for you.

I mention Avast specifically because it's most known for doing this, but other security programs can have a similar setting that also causes the same issue.

Hope this helps.

I had seen others mention Avast being their problem, but I am using Kaspersky Internet Security. What's odd is that the problem is only on startup. It never occurs again after the first 5-10 seconds of loading the browser.

Opening additional windows (regular or private) also populate instantly as long as I had a browser open already.

If you disable your browser extensions, so that fix the problem? It's possible that an extension (I'd suspect Adblock Plus) is either blocking content from loading until the extension has finished starting or hogging Firefox resources while it starts.

Disabling all extensions and even using a brand new profile do not resolve the issue. My browser is currently working normally, but if I were to close it and re-open it, I would experience the initial hang on startup but never again for the remaining lifetime of the firefox.exe process.

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

Boot the computer in Windows Safe mode with network support to see if that has effect in case security software is causing problems.

• https://www.bleepingcomputer.com/tutorials/how-to-start-windows-in-safe-mode/
• https://www.7tutorials.com/4-ways-boot-safe-mode-windows-10

I disabled the firewall entirely and also disabled my internet security. Nothing changed. Again, this is only on firefox startup. I can browse without any lag right now.

I will try safe mode tonight.