”Webpage is slowing down your browser” This is the new way the browser tells you there is a problem with the script program(s). https://support.mozilla.org/en-US/kb/warning-unresponsive-script?cache=no

https://support.mozilla.org/en-US/kb/task-manager-tabs-or-extensions-are-slowing-firefox

You can try to clear Firefox’ Browsing Data

If a corrupt file managed to find its way on your computer via the browsing data Firefox uses, it can be expected to see some change in performance. This can only be fixed by getting rid of this file so make sure you do so by deleting the Firefox’ browsing data such as cookies, temporary files, etc.

1. Open your Mozilla Firefox browser by double-clicking its icon on the Desktop or by searching for it in the Start menu.

2. Click on the library button located at the top right part of the browser’s window (left from the menu button) and click on History >> Clear Recent History…

3. There are several options for you to set. Under the Time range to clear option, choose Everything by clicking the arrow which will open the dropdown list.

4. Click on the arrow next to Details where you can see just what will be deleted when you select the Clear history option. History on Firefox means a lot more than it does on Chrome since history in Firefox includes cookies, temporary data, etc.

5. We recommend you choose at least the Browsing & Download History, Cookies, Cache and Active Logins before you click on Clear Now. Wait for the process to finish and restart your browser. Check to see if the problem is gone now.

There are plenty of method to solve this problem, but I used the method above to solve the problem. You can refer to this link: https://appuals.com/fix-a-webpage-is-slowing-down-your-browser/