The purpose of reloading web page(s) and bypass the cache is to get a fresh copy of possibly outdated or corrupted files currently stored in the disk cache. You can do this as the first attempt if a normal reload doesn't seem to be giving an up to date version of a website or if you otherwise experience issues with a website. We usually advice this in case of issues:

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Reload web page(s) and bypass the cache to refresh possibly outdated or corrupted files.

  Hold down the Shift key and left-click the Reload button
  Press "Ctrl + F5" or press "Ctrl + Shift + R" (Windows,Linux)
  Press "Command + Shift + R" (Mac) 

Clear the cache and remove cookies only from websites that cause problems. “Clear the Cache”:

  Firefox/Tools > Options > Advanced > Network > Cached Web Content: "Clear Now" 

“Remove Cookies” from sites causing problems:

  Firefox/Tools > Options > Privacy > "Use custom settings for history" > Cookies: "Show Cookies" 

Start Firefox in Safe Mode to check if an extension or theme (Firefox/Tools > Add-ons > Extensions) or if hardware acceleration is causing problems.

• Difference between "Reload" and "Reload (Override Cache)"?