You can find out which tabs and addons are using resources with Task Manager (Menu Bar > Tools > Browser Tools > Task Manager or 3 bar menu > More Tools > Task Manager). This article may help: https://support.mozilla.org/en-US/kb/firefox-uses-too-much-memory-or-cpu-resources

Hi, it's more likely the disk cache that would be using gobs of space, and not the cookies and site data.

Try keeping the cookies and site data, but clearing the "Cached Web Content" and see whether that helps. That's one of the two checkboxes when you click the Clear Data button on the Settings/Preferences page.

If you have a high bandwidth connection, you can turn off the disk cache to save on disk space. Here's how if you want to try it:

(1) In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button accepting the risk.

(2) In the search box in the page, type or paste browser.cache.disk.enable and pause while the list is filtered

(3) Double-click the preference to switch the value from true to false

More info on about:config: Configuration Editor for Firefox.

You can create a new profile as a quick test to see if your current profile is causing the problem.

See "Creating a profile":

• https://support.mozilla.org/en-US/kb/profile-manager-create-remove-switch-firefox-profiles

If the new profile works then you can transfer files from a previously used profile to the new profile, but be cautious not to copy corrupted files to avoid carrying over problems.

• https://support.mozilla.org/en-US/kb/recovering-important-data-from-an-old-profile