There are two dictionaries on my system that contain almost identical words & spelling corrections but are stored in different locations and maintained separately.
Words like "PowerPoint", "Gmail", "AdSense", "YouTube", etc. exist in both dictionaries but one is created through Firefox while Windows Live Writer is responsible for the second one.
That means lot of duplicate effort because if you add the word "Facebook" to Firefox dictionary, it would still be marked as incorrect inside Windows Live Writer.
There’s no easy way to sync dictionaries but as they are plain text files, you can manually transfer words corrected in Firefox to Windows Live Writer dictionary or vice versa.
The dictionary specific files are persdict.dat (Firefox) and userdic.tlx (Windows Live Writer). Their location is:
Location of Windows Live Writer Dictionary:
Windows Vista – %AppData%\Windows Live Writer\Dictionaries\userdic.tlx
Windows XP – C:\documents and settings\<username>\Application Data\Windows Live Writer\Dictionaries\userdic.tlx
Location of Firefox Dictionary:
For Windows Vista – %AppData%\Mozilla\Firefox\Profiles\xxx.default
For Windows XP – C:\documents and settings\<username>\Application Data\Mozilla\Firefox\Profiles\xxx.default\
Firefox stores all corrections and new words in a separate line in the order in which they were inserted. Windows Live Writer adds a tab followed by the symbol "i" after every word and arranges them in alphabetical order.
Just keep the above point in mind and start editing. For instance, you may copy all text from Firefox dictionary and paste it inside userdic.txt of Live Writer. Then add a tab + "i" after every word and you won’t see a red dotted line beneath them in WLW.
If there are too many entries in your Firefox file, open that inside Excel and use the CONCATENATE function to make them Live Writer compatible.