Might be a bug, so we can troubleshoot to try to figure it out.
If you visit about:processes, is there any website or extension taking up a lot of system resources?
In addition to that, you can try launching firefox in safe/troubleshooting mode. 3 bars in the top right > Help > Troubleshooting mode
This restarts firefox with all addons, extensions, and themes disabled.
For further troubleshooting, also try:
Go to about:profiles, and create and launch a new one. This is a more complete reset, since it also resets all settings, about:config options, and similar.
After creating a new profile, you can play around with rebuilding your current firefox config, adding extensions and changing settings.