Start Firefox in Safe Mode {web link} by holding down the <Shift> (Mac=Options) key, and then starting Firefox.

A small dialog should appear. Click Start In Safe Mode (not Refresh). Is the problem still there?

https://support.mozilla.org/kb/Firefox+is+already+running+but+is+not+responding

https://support.mozilla.org/en-US/kb/firefox-slow-how-make-it-faster

https://support.mozilla.org/en-US/kb/firefox-uses-too-many-cpu-resources-how-fix

https://support.mozilla.org/en-US/kb/firefox-uses-too-much-memory-ram

https://support.mozilla.org/en-US/kb/firefox-hangs-or-not-responding

Unfortunately this didn't help, and I don't really understand how it actually could. I just downloaded it to test it, I'm using the latest Mac OS High Sierra, and the latest version of Firefox. It's much less responsive and doesn't perfom well compared to other browsers. Everything it does, it does considerably slower. Watching 720p videos on YouTube is almost impossible, as it's choppy, I looks like the browser is really struggling to process it. When I open the same video in Chrome, it strarts fast and just plays smoothly without problems.

I'm aware of the fact that I'm using almost 9 year old computer, but If Chrome works fast, why Firefox is so slow? I remember long time ago having Windows XP, and I would ony use Firefox back then, it works perfectly, but on a Mac, I get the feeling that this browser simply doesn't go well with Apple systems. Weird...

https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-using-safe-mode

Safe Mode is a troubleshooting mode that temporarily turns off hardware acceleration, resets some settings, and disables add-ons (extensions and themes). If the problem goes away, that tells us one thing. If it does not, that tells us something else.

Start your Computer in safe mode with network support. Then start Firefox. Try Secure websites. Is the problem still there?

http://encyclopedia2.thefreedictionary.com/Linux+Safe+Mode Starting Any Computer In Safe Mode; Free Online Encyclopedia

It's not a solution. There are no add-ons and no themes, it's a clean install of the latest Firefox! It has never been faster, it's always been extremely slow. I've been trying Firefox for years once in a while, only to throw it to Trash minutes after installing. It's always been like an elephant. I tried it on my MacBook Pro 17' Mid 2009 and also on a 13" MacBook Pro Mid-2010. I tried it year ago, two years ago, 6 years ago and it was always the same! I was never able to watch a 720p video on YouTube in Firefox! Ever. Chrome and Safari on the same computers work perfectly, Chrome being the fastest so far.

I was just hoping that something improved over the years. I also spoke about this with some of my friends who are web developers and front end developers, they all say the same thing! They say they use Firefox occasionally for testing, that's all. All on Macs, I'm not sure how is the experience on a PC.

There's a few things you can try:

• First, double check that you are using a completely clean profile. You said you used to use Firefox, so it may be trying to use an old profile. Profile Manager - Create, remove or switch Firefox profiles The profile you posted this question with has a lot of add-ons in it.
• Then, double-check that your Mac is fully up to date. The latest version of Mac is 10.14.1
• After that, see if Hardware Acceleration is enabled. If it is, try disabling it. Turning off hardware acceleration
• I'd suggest making sure you have no malware on your machine as well, run a quick scan with www.malwarebytes.com

Let me know if this helps!

Tyler Downer said

There's a few things you can try:

• First, double check that you are using a completely clean profile. You said you used to use Firefox, so it may be trying to use an old profile. Profile Manager - Create, remove or switch Firefox profiles The profile you posted this question with has a lot of add-ons in it.

I'm using a clean profile. I don't use any add-ons. I did use Firefox in the past, long time ago on Windows XP.

Tyler Downer said

• Then, double-check that your Mac is fully up to date. The latest version of Mac is 10.14.1

I'm on the latest version of High Sierra. My MacBook Pro is not compatble with Mojave. I tired Firefox on Sierra, it was slow as hell. I tried on El Capitan, it was slow as hell. I tried on Yosemite, it was slow as hell. I tried on Maverics, it was slow as hell. I tried on Mountain Lion, it was slow as hell. I tried on Lion, the same. I tried on Snow Leopard and Leopard - it's always been slow compared to other browsers on a Mac. Not my observations only, many of my friends are saying the same thing.

I'm a graphic designer and front end developer, and I always have a clean version of Firefox which I use for testing, but that's about it. I cannot use it for anything else, because it is painfully slow.

Tyler Downer said

• After that, see if Hardware Acceleration is enabled. If it is, try disabling it. Turning off hardware acceleration

Didn't help. I don't know what you mean by "upgrade your graphic drivers to use hardware acceleration and WebGL".

Tyler Downer said

• I'd suggest making sure you have no malware on your machine as well, run a quick scan with www.malwarebytes.com

No malware. It's a clean MacOS High Sierra system with some graphic design software installed.