You were trying to install a Language Pack which is for to change the language of the UI or user Interface of Firefox as the inline spellchecker uses Dictionaries.

On top of https://addons.mozilla.org/firefox/language-tools/

Installing a dictionary add-on will add a new language option to your Firefox spell-checker, which checks your spelling when you fill out Web forms. Language pack add-ons change the language of the entire application.

As you can see, two different things. https://support.mozilla.org/en-US/kb/how-do-i-use-firefox-spell-checker https://support.mozilla.org/en-US/kb/use-firefox-interface-other-languages-language-pack

Firefox 61.0 is going to be Released on morning of June 26 Pacific time. The last couple major Releases had the Languages Packs ready for the new major Release a day early instead on same day or late a day.

Yes i try install Language pack and have next message:

"This add-on requires a newer version of Firefox (at least version 61.0). You are using Firefox 60.0.". Yes dictionary helps me, but if i want install Language Pack - how to i do it, if I have FF60, but need 61?

https://addons.mozilla.org/en-US/firefox/addon/russian-ru-language-pack/

You asked for how to have Russian for the spell checker, in which you needed a Russian dictionary.

However if you want to have one Firefox install but be able to change the user interface of Firefox menus and Preferences between two or more languages then you can install a Language pack(s) and toggle between languages as explained in https://support.mozilla.org/en-US/kb/use-firefox-interface-other-languages-language-pack

As for Russian Language pack for say Firefox 60, look at the bottom of the page for "Version Information > See complete version history" which is https://addons.mozilla.org/firefox/addon/russian-ru-language-pack/versions/