Your page doesn't look it will support Reading mode.

"Reader View" (about:reader?url=) is only available on webpages that meet certain conditions like a specific layout. There is a minimum character count for containing elements that show in Reader mode, so you may not see all the text if the page isn't designed to be used with Firefox Reader mode.

• view-source:resource://gre/modules/reader/Readability.js

There is a general purpose add-on you could look at called Read Aloud, but according to one review, it only reads English (?):

https://addons.mozilla.org/firefox/addon/read-aloud/

What language version of Firefox are you using? That is Russian site And how are you doing the Read aloud in Edge? You have to provide steps your doing.

Preparing file to be read aloud for MS Edge: The simplest way is to type the text in Notepad, using both English and native language, save it, change the extension (txt to html) and open it in MS Edge (right button click > 'Open with' > MS Edge), start Read aloud (Ctrl+Shift+U) - it reads only english words; I stop it and change 'Voice parameters' to one of MS <native language> on-line readers. Now it reads also native language words. But it's better to use an HTML editor for typing, because after Notepad, the browser shows the text as one line.

Also that sites isn't loading in Firefox as well. So if the site can't load ReadAloud won't work. Also Edge in Windows 10 is loading so the ReadAloud if feature won't work either.

The site is opening both in Firefox and in MS Edge, in MS Edge the page is reading aloud. May be the site is blocked on Your device as it is placed on free hosting.

As I understand, MS Edge Read aloud function uses powerfull MS Online readers providing many native languages. I tried it for voicing of a technical video. Result is very good and MS Edge is still installed.