Need help with BF Cache

[pedz]

Here is the mystery: one item FF 26.0 decides to cache for 3 months (call this item 1).  Another item it decides not to cache (call this item 2).  I'm trying to figure out why.  Both of these are javascript files.

If I dump the cache entirely and reload, I get fresh loads of the two items.  Looking at the Net panel of the two items, looking at the response header both "view source" and "pretty print" I don't see any difference between the two responses except for "Last Modified".  For item 1, last modified is April 2012.  For item 2, last modified is a few minutes ago.

Am I looking at all of the response headers or are there more hidden somewhere that I'm not aware of?  Does FF decide when to expire items in the cache based upon the last modified date?

The real practical problem is sometimes these items can change and FF doesn't know it.  I'm sure it is my fault but I can't figure out where or how I'm making the error.

Thank you for your help,
Perry