Hi, and thank you for asking your question.
If you are using Firefox as your browser, ending the "Firefox.exe" process will close your active Firefox windows itself. However, if you need to reduce the amount of memory resources taken up by Firefox, please do the following:
In Firefox, click on the orange Firefox tab at the top-left, then click Help > Restart with Add-ons Disabled. Firefox will now close and a pop-up window will come up. Check all of the options listed there, EXCEPT for the option to delete your bookmarks, then click 'Save Changes and Exit'. Now, when you open up Firefox, it will run much faster than usual, and will not take up as much memory resources as before, as well.
Also, regarding the general speed and functioning of your computer, there are a few things I would like you to carry out as well:
1) First off, please go to Start > Run, and type in msconfig
in the search box, and press ENTER.
In the System Configuration Utility window that comes up, click on Services, then check 'Hide All Microsoft services', then click 'Disable All'. Done that, click on Startup, then disable all of the entries listed, except for the ones you need to use, e.g. for your security program. Hit Apply thereafter, and restart the computer accordingly.
2) Next, please download the following freeware (don't buy anything) security program, Malwarebytes' Anti-Malware
Download, run and install the program, then carry out a quick system scan using the same, and remove the security threats detected, if any.
3) Finally, there's a handy freeware utility I would suggest you to download, as follows: http://www.piriform.com/ccleaner/download/standard
Install and run the same and click on the 'Run Cleaner' button in the window that comes up in order to do a comprehensive cleanup of useless data in your system.
Next, click on the 'Registry' tab present in the left pane, then scan for and fix issues found in your registry.
This should do it. Hope it helps. Thank you!