Does Windows show "NOT RESPONDING" in the Firefox title bar? Please see whether anything in the following article is useful:

Firefox hangs or is not responding - How to fix

If you get any other kind of indication of the problem, could you provide more detail about it?

One of the innovations in Firefox 48 is that some users have a new feature enabled, which separates the browser interface process from the page content process. The performance impact of this can vary a lot between systems, so could you check whether you have this feature turned on? Either:

• "3-bar" menu button > "?" button > Troubleshooting Information
• (menu bar) Help > Troubleshooting Information
• type or paste about:support in the address bar and press Enter

In the first table on the page, check the row for "Multiprocess Windows" and see whether the number on the left side of the fraction is greater than zero. If so, you are using e10s, and it may be performing poorly on your system...

If you need to turn off e10s, you could try this:

(1) In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button promising to be careful.

(2) In the search box above the list, type or paste autos and pause while the list is filtered

(3) Double-click the browser.tabs.remote.autostart preference to switch the value from true to false

That will most likely take effect the next time you exit Firefox and start it up again.