Assume that you have checked your Total HDisk usage and it is not full or has at least has a few GB free?
It's feasible that Firefox is causing the problem and a reinstall may provide the fix.
If not, Ubuntu includes a 'System Monitoring' application tool with which you can monitor a number of system resources and is very similar to the Microsoft Windows 'Task Manager'.
To start the 'System Monitor' from the Panel menu select:- System, Administration, System Monitor
A useful write up on how to use the tool can be found at:
http://www.omgubuntu.co.uk/2011/11/5-system-monitoring-tools-for-ubuntu
Initially check the CPU usage graph to see if it is frequently or constantly peaking at 100%. If so, then check each of the running Processes to determine which process is causing this condition.
A further possibility to consider is that there may be some corruption to the system files. To check this you could use a Ubuntu system program called 'fsck', but you do need to be very careful in its use. See the following:
https://www.maketecheasier.com/check-repair-filesystem-fsck-linux/
Hope that helps.