The dictionary tooltips add-on is one of my favourite Firefox extension, But it fails to work after my Firefox update to 22. Since the add-on author may need a bit of longer time to release a official update.
NOTE: the add-on is not under an open source license , so using the following process is your risk.
Step 1
Go to your Firefox profile folder, am using windows 7 ( if you are using linux operating system then goto the Firefox profile folder in that OS ) so the profile folder is located in
C:\Users\{your username}\AppData
If you are not able to see AppData then you must enable show hidden files and folders option, to enable this feature do the following
Click Organize (Top left corner) –> Folder and search options –> View (Tab) –> Show hidden files, folders and drive (Radio button) –> Click Ok
Now you can see AppData inside C:\Users\{your username}\ folder, then move to the following folder
C:\Users\{your username}\AppData\Roaming\Mozilla\Firefox\Profiles\{profilename}.default\extensions\