hello, i think that the cache in firefox is primarily designed to achieve faster page load times by caching often used or large web site elements & not for offline use. you could try if you can access such pages when you go to firefox/tools > web developers > work offline.

as an alternative you could use an addon like ScrapBook in order to save and organize sites for offline use...

It seems like FireFox 21 never loads from cache - even big pages that I use every session appear in their current (live) version. FireFox 3 could be set to load from cache first, and go live via "refresh". Of course I can "save file as" on my local hard drive to look at later (and I do). But isn't it ridiculous if the History list can't find that page in its own cache?

    (by the way, "Work Offline" is under the "File" menu)

Yes, if the menu bar is visible then you find Work Offline in the File menu.
If you have the orange Firefox menu button then you can use Firefox > Web Developer > Work Offline

You can find the location of the disk cache on the about:cache page.

In case you are using "Clear history when Firefox closes":

• do not clear the Cache because Firefox will disable the disk cache in such a case

• Tools > Options > Privacy > Firefox will: "Use custom settings for history": [X] "Clear history when Firefox closes" > Settings
• https://support.mozilla.org/kb/Clear+Recent+History

Note that clearing "Site Preferences" clears all exceptions for cookies, images, pop-up windows, software installation, and passwords.

The offline (application) cache is used by other applications to store data and serves a different purpose.

• https://developer.mozilla.org/en-US/docs/IndexedDB
• https://developer.mozilla.org/en-US/docs/HTML/Using_the_application_cache