Did you know that Firefox comes with a built-in task manager? That's right. And with this task manager, you can see which of your tabs are causing your system to slow down. This is an incredibly important (and underused) tool in the Firefox tool box. You can see how to access and use it at https://www.techrepublic.com/article/how-to-use-the-firefox-task-manager/.
A further useful tip related to this is to set Firefox to automatically suspend background tabs if memory resources are low. To do this go to about:config and search for lowmemory and change to true.
With the help of the Firefox Task Manager, you can find out which tabs are consuming too many system resources.