Look in /var/cache/apt/[somefolder] and you should be able to find the debian file for FireFox 28. Install that.
Then you'll need to blacklist firefox in apt so it doesn't update.
Note: Never used LMDE, so I'm not sure of the exact details, but that's pretty much how you do it.
Problems? Tell us EXACTLY what you've done and what you expected to happen, IN DETAIL. That will save us questions, and we should get along better,
I have dysgraphia. This means I might have understood you incorrectly through no fault of my own.