Mozilla firebird stores bookmarks in
<your home directory>/.phoenix/default/<some random value>/bookmarks.html

If you cannot find it just run the command to find the file named bookmarks.html and you will get the file.
The exact steps you can follow is
1>cd to your home directory
2>run the comamnd "find . -name bookmarks.html -print"
It is a regular html file.

You can even import the file in your current install of Mozilla-Firebird.
Hope this helps.