HI, please see this page : https://addons.mozilla.org/en-GB/firefox/addon/dictionnaire-fran%C3%A7ais1/

It appears that you are trying to install a Language Pack and not the Dictionary.

See the description of the two on top of https://addons.mozilla.org/firefox/language-tools/

The Language Pack is use for changing the language of the User Interface or UI while the Dictionary is for the inline Spellchecker so it has a pack of words to spell check with like well a dictionary book.

Unless otherwise I thought the Français Firefox comes with a dictionary as only a few locales do due to license reasons.

The Français dictionary is at https://addons.mozilla.org/firefox/addon/dictionnaire-français1/

This article describes what and how the Language packs are used. https://support.mozilla.org/en-US/kb/use-firefox-interface-other-languages-language-pack

It is basically a option for if you want only one Firefox install from www.mozilla.org/firefox/all/ but be able to switch the between two or more different languages for the UI.

The Firefox 62.0 is not scheduled for Release until September 5th so it is a bit early as the change for the new Language Pack version should be a day early at most normally.

If you do want to install the Français language pack for Firefox 61.0.2 then scroll down https://addons.mozilla.org/firefox/addon/français-language-pack/ and look for Version History See all versions link which is https://addons.mozilla.org/en-US/firefox/addon/fran%C3%A7ais-language-pack/versions/

The previous Version 61.0buildid20180807170231 is for Firefox 61.0.*