To "clear cache" on Firefox, look for the three bars (or hash marks) in the toolbar at the upper right of your display.  Any page will do.  You do not have to be on an Agoracom page.

Click the bars to open a menu and look for "History."  Click that to open another menu.

Find "Clear recent history" and click it.  In the box that opens, click "Cache" to enter an "X" there, and then click "Clear now."  You're done.  Easy peasy! 

WARNING - Be certain to enter an "X" only at the "Cache" option in the last step.  If "Xs" appear anywhere else, remove them.