In whatever browser you use (like Firefox or Chromium or Chrome or whatever) make sure to clear the cache. In Firefox, it's found at History/Clear Recent History... <-- this gives the option to clear all your history (you may wish to preserve cookies and logins, but the cache should be cleared.)
Also, run the following commands in your terminal/console:
sudo apt autoclean && sudo apt autoremove
Note, if these commands do not work, then try what's below. The commands above are the newer updated commands in Ubuntu, though maybe Mint still uses the older "apt-get" command. If so, try what's below:
sudo apt-get autoclean && sudo apt-get autoremove
This should free up some space. Some of the other suggestions made before by those with more knowledge than I are likely also helpful. Good luck.