Hi, the cache default is set to 350megs. As the cache fills as you visit sites it goes to full at 350megs. So once full it will delete files and new ones and the cycle keeps going automatically. Yes it is good to clear the cache. Especially if working with web pages as the cache holds images and more so when yo make changes to a document or webpage the changes sometimes can not be seen till you clear the cache. As for Firefox very slow it has nothing to do with the cache though clearing it will help temporarily. It just shows here that you have version 59 installed, current version is 59.0.2 suggest update if not running that. You can find out from Help --> About You do not have send info to Mozilla turned on with in Firefox so I do not know various things that would be useful like issues with video card drivers and what Extensions/add-ons you are using. Also Video card will let me know last time it was updated and what card it is which could help me find drivers for you but it also gives me a idea of how old your system is and how to help a aging system. Add-ons take about 20megs each to run fyi. You can try these see if anything helps : *https://support.mozilla.org/kb/Firefox+is+already+running+but+is+not+responding *https://support.mozilla.org/en-US/kb/firefox-slow-how-make-it-faster *https://support.mozilla.org/en-US/kb/firefox-uses-too-many-cpu-resources-how-fix *https://support.mozilla.org/en-US/kb/firefox-uses-too-much-memory-ram *https://support.mozilla.org/en-US/kb/firefox-hangs-or-not-responding If not you can try this : Please Refresh but do this 1st: Delete your Cookies and Cache and TEST. *https://support.mozilla.org/en-US/kb/export-firefox-bookmarks-to-backup-or-transfer *https://support.mozilla.org/en-US/kb/back-and-restore-information-firefox-profiles then *https://support.mozilla.org/en-US/kb/refresh-firefox-reset-add-ons-and-settings TEST....... If no issues then Extensions which need to be added back in 1 at a time and tested ..... Please let us know if this solved your issue or if need further assistance.