What's up with spell check in firefox 39?

Question

Browsers

Spell checking is enabled but it works very strange, for example, it works fine here, but in VK or Google mail, everything is underlined in red
in the Yandex browser, spell checking works everywhere, but it loves RAM too much

Answer 1

Noticed this feature. The fact is that for some reason the dictionary sometimes switches to English, so he underlines all Russian words in red. It is necessary to switch back to Russian in this case. It was not possible to find any pattern, although I did not particularly look for it.