Well, there can be more than one profile in Firefox, and you want to find the one you usually use. When you're looking at the results of your Windows search, do so in "details view", and have it sort the results by date. The most recent ones should be from the last time you used Firefox, so the folder those are in is the one for your usual profile. Copy a bookmarks file to that directory, and rename it "bookmarks.html" - do this while Firefox is not in use. When you start Firefox, that new bookmarks file should be the one it uses.

When you're looking for likely candidates among the results of the Windows search, pay attention to the file size. The default bookmarks file that comes with Firefox is tiny; my regular working bookmarks file is >400KB in size.

This is a manual way of replacing the entire bookmarks file. You might want to try the method that justinwrock has offered to see if that more conveniently gets you where you want to be.