From MozillaZine Knowledge Base
The file "history.dat" stores the browsing history (a record of visited pages) in Firefox 2 or below and in Mozilla Suite/SeaMonkey 1.x. (Starting in Firefox 3 and SeaMonkey 2, history.dat is no longer used and the browsing history, as well as bookmarks in Firefox 3, is stored in the file places.sqlite.)
Browsing history should not to be confused with session history which controls the Back and Forward buttons, or with download history which stores details of files saved to the hard drive or opened with external programs. The History Sidebar provides an interface to view and edit this file.
By default, the browsing history is stored for 9 days. If the history becomes long enough, the file can become large enough to greatly increase the startup time. To reduce the startup time, you can delete all or part of your browsing history, or you can delete the file.
The "history.dat" file is written in a complex format called "Mork". Changes can be made through the History Sidebar, but editing this file manually is not recommended.
This file can be moved to a different profile.
You can delete your history from the main menu (Tools => Clear Private Data...). If you suspect that the file is corrupted, deleting "history.dat" will delete your browsing history, and a new, empty file will be created automatically.
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