Hello Marat,

This article should help:

Change what Firefox does when you click on or download a file

RobertJ said

Hello Marat, This article should help: Change what Firefox does when you click on or download a file

Нет, к сожалению, эта статья не помогла.

You can reset your Applications handler.

Type (or paste) about:support in the address bar and press Enter under Application Basics next to Profile Folder click the Open Folder button Close Firefox.

In your Profile Folder rename handlers.json to something like handlersOLD.json If you see a file named mimeTypes.rdf, rename that one to mimeTypesOLD.rdf

Close the folder window and launch Firefox. Your Applications handler will be reset to the default. You can set how to handle PDF's and set how to handle other file types as you encounter them.