See:

• https://support.mozilla.org/en-US/kb/clear-cookies-and-site-data-firefox
• https://support.mozilla.org/en-US/kb/how-clear-firefox-cache

"Remove the Cookies" for websites that cause problems:

• Settings -> Privacy & Security
  Cookies and Site Data: "Manage Data"

"Clear the Cache":

• Settings -> Privacy & Security
  Cookies and Site Data -> Clear Data -> [X] Cached Web Content -> Clear

Computer: Open Firefox on your computer. In the upper right-hand corner of the browser, click the “menu bars,” which look like three parallel lines, and click the “Privacy” tab. Here you can select “Clear your recent history.” Be sure that only “Cookies” is checked, and then select the time frame for which you would like to delete your cookies. If you want to delete all cookies, select “Everything.” Double check your selections to make sure that other items you want to keep are not selected, and then hit “Clear now.”

Android: Some instructions may vary based on the type of phone you have and what version of Firefox it is running. Look for the menu icon (three bars) located at the top right corner. On older Android devices you’ll have to press the hardware menu key and then tap “More.”

Go to the “Settings” menu. On the settings menu, look for “Privacy & security” and select “Clear private data.” You will then be taken to a list of what can be cleared where you can select “Cookies & active logins.” After you have made your selection, tap the “Clear data” button to finalize deleting cookies.

IOS: Open Firefox. Click on the “menu bars,” which looks like three parallel lines in the lower right-hand corner of the browser window. Select “Settings.” Scroll down to “Privacy.” Select “Clear private data.” Make sure only “Cookies” is selected, then tap on “Clear private data.”