Deleting cache trash OK, outside of Firefox?

Hi again All, Prior to doing the complete system backups (as outlined in my last email), I enter Firefox and clear all history. But, during the backup (after booting into Recovery mode), rsync displays the copying of MANY files within this directory... /home/user/.cache/mozilla/firefox/abcd1234.default/cache2/trash/3363925 Given the names of some of those directories, I wonder if it is safe to delete them (rm -r) just before the backup? If so, at which level... trash? cache2? Thanks very much, in advance. Carl Turney Bayswater, Victoria