The Greek Firefox version comes with its own builtin dictionary as you can see if you open this link via the location/address bar (GRE means: Gecko Runtime Environment).

• resource://gre/dictionaries/

You can't disable such a builtin component. You would have to install another Firefox locale, either one that comes with your preferred dictionary if there is such a locale (en-US) or one that doesn't come with a dictionary. You can install the Greek language pack if you prefer a user interface in Greek. You can install a spell check dictionary of your choice.

Do not confuse a language pack and a dictionary as both are listed on the same page.

• https://addons.mozilla.org/firefox/language-tools/

@cor-el thanks for your answer. Maybe you don't give me the answer that I was looking for but some of your hints gave me new ideas and I found a solution to solve my problem

I would like to disable it because sometimes (especially after updated) firefox swap to default

In order to install Firefox with your desire dictionary and desired language you have to install a Firefox version without it. This version is en-GB https://www.mozilla.org/en-GB/firefox/new/

In order to install property the firefox en-GB you have to follow those steps bellow

1. Through windows Control Panel unistall firefox
2. Press window key (between left Ctrl and Left Alt)
3. Type %APPDATA%
4. Navigate to Mozzila Forlder and delete of rename firefox to firefox.old
5. Install en-GB version your download previously
6. Add Greek language interface and English - Greek dictionary
7. Go to settings -> General -> (Section) "language and Appearance" -> Language -> Through drop down menu choose Greek

Warning 1 : If you don't have an firefox account you will loose all your setting, add-ons, bookmarks etc at steps 3 and 4. If you don't wish to make an account you have to find a way to move them in the new installation. You can't skip this steps because old dictionaries will reappear in new installation

Warning 2 : Do not skip the step 6 and try to add Greek language through "Search more language". This will add Greek dictionary also. You have to install Greek interface through firefox add ons

In near future firefox should reconsider the way that handles dictionaries. If I want to type in Greek - English - Portugal I have to install 3 dictionaries and choose between them. This is time consuming and its confusing if you type simultaneously 2 or more languages. Other browsers like chrome searching at the same time more than 2 dictionaries if user desire it and you don't have to install hybrid dictionaries with more than one language (eg English - Greek)