Firefox Clearing Cache - Ctrl + F5 doesn't work randomly
How to get around the issue
If Ctrl-F5 isn't working, Ctrl-Shift-R works as a fine substitute to reload the page without the cache.
Why this happens
The Ctrl-F5 key event may be captured by your OS, and so Firefox never sees it. For example, XFCE captures Ctrl-F5 key events, and tries to switch to the fifth workspace.