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Perishable Press

How to Edit the Firefox Custom Dictionary

Oooops! Didn’t really mean to add that particular word to the Firefox custom dictionary. Better remove it now before it causes problems later on..

[ Firefox ] As one who takes full advantage of the custom dictionary in Firefox, I occasionally find myself adding nonexistent or misspelled words to the dictionary by accident. Not wanting to deal with a false negative down the road, I always take the time to stop what I’m doing, locate the custom dictionary, and remove the erroneous term. Finally getting sick of trying to remember the esoteric location in which Firefox stores the personal dictionary, I decided to make a few notes and post the information here for easy access when it happens again.. and it will happen again, there is no doubt ;)

Turns out that the location of the custom dictionary varies depending on your operating system. The name of the custom dictionary is “persdict.dat” (short for “personal dictionary”), and is located in the following location (depending on operating system):

Windows Vista


Windows XP

C:\Documents and Settings\[username]\ApplicationData\Mozilla\Firefox\Profiles\[unique-alphanumeric-string].default\persdict.dat

Mac OS X

~/Library/Application Support/Firefox/Profiles/[unique-alphanumeric-string].default/persdict.dat



Of course, the [username] (for Windows) and [unique-alphanumeric-string] will vary depending on your unique configuration. Once you have located the target file, “persdict.dat”, open it with a text editor and edit according to your needs. The file itself is simply a list of words that you have chosen to add to the dictionary by right-clicking in Firefox and selecting “Add to dictionary”. With this file, you can add as many custom words as you like, or, conversely, remove all those nefarious little rascals that you added by accident! Save, close, and done. Even so, why hasn’t somebody written an official “custom dictionary editor” extension for Firefox? Would be so nice!

Jeff Starr
About the Author Jeff Starr = Designer. Developer. Producer. Writer. Editor. Etc.
11 responses
  1. Hi Perishable.
    I think that this hack (or trick or tweak or whatever you’d like to call it) fits with mine :)

    I don’t like spam :) but this arcticle reminds me my “Custom Firefox favicon bookmark toolbar” that you can find at

    I’d like to think (and tell me if it’s so) you’re just read it.


  2. Jeff Starr

    Yes, I read your article, and it is very good. It seems like a very useful technique. I think I will experiment with it a bit and examine the results (just as soon as I can find the time!;) Thanks for sharing it with us! :)

  3. Hey
    thanks for the tip but i tried to change to words in the persdict.dat file and then saving the file. but it doesn’t work. when i open the file after using the dictionary once the old words are back, and the new ones have disappeared!

  4. AH! sorry for previous comment! I just realised that I had forgotten to close down Firefox when I was changing the file!
    Thanks for the tip :-) It does work!

  5. I just was asking for that! :)

  6. Jeff Starr

    Thanks for the update, me! Glad to hear you got it working :)

  7. I don’t have that file on my computer, i’m using xp and i know my way around computers, i searched for it by name and still couldn’t find it, I also looked in every available profile setting (Admin, All Users, etc…) and it’s nowhere to be found… I use the built in dictionary constantly, so could the file be named something different?

  8. Jeff Starr

    Hi matt, as far as I know, persdict.dat is the standard name for the dictionary file on XP. If a thorough search does not return any results, then something may have been customized, overwritten, or otherwise modified by another program. You may want to check around at the Mozilla Firefox forums and maybe ask if anyone has any ideas.

  9. Simon Field May 11, 2009 @ 6:45 am

    Thanks for that – I’m forever hitting the “Add to Dictionary” option by accident, and didn’t feel like trawling through the ApplicationData for myself.

  10. Jeff Starr

    Absolutely my pleasure, Simon. That’s exactly why I posted this article — I am always clicking the wrong button as well and got tired of trying to remember the location of the custom dictionary. Another thing that I did to make it even easier and save more time is create a shortcut to the dictionary file itself. Now I am good to go with a single click!

  11. Thank you so much for this! It helps a ton.

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