You can use Ctrl + F5 for a single use basis, or if you want firefox to always reload the page then type this into the address bar:
about:config
type this into the filter:
browser.cache.check_doc_frequency
change the value to: 1

more info:
3 - only check if it seems outdated (which is default)
2 - always use cached version
1 - always check for newer version
0 - check for newer version once per session