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You'll find it in the All Apps area of the Start menu in Windows, and in the Applications folder in macOS. When you use Firefox's Safe Mode, you'll be starting up a clean version of Firefox that doesn't use any Add-ons (extensions or themes). If using Firefox is faster when you're in Safe Mode, the issue is probably with an installed add-on or theme. It's at the top-right corner. It's near the bottom of the menu. This option appears as a ? icon on some versions of Firefox. A confirmation message will appear. A message with information about Safe Mode will appear. Firefox will now launch without extensions and themes. If it's much faster to use Firefox in Safe Mode, it's probably because one of your add-ons is acting up. See Disabling Add-ons to learn how to turn these features off. Start by turning all of them off. Then, enable just one add-on and try browsing with it. If browsing is still nice and fast, you can leave that add-on enabled and try another. Keep enabling add-ons until you've found the one that's causing problems.
Open Firefox on your PC or Mac. Click the menu. Click Help. Click Restart with Add-ons Disabled. Click Restart. Click Start in Safe Mode. Browse the web.