I'm using several versions of firefox: nightly, developer, stable.
There is a problem however:
Or more precisely:
https://addons.mozilla.org/en-US/firefox/addon/english-us-language-pack/
As I'm using more than a single language on a daily basis, this is a problem (it highlights words which are correct and I cannot select proper dictionary). In fact I would like to mark a text that is is a mixture of 2 or more languages. Or mark its parts to be in a given language. But that is not possible with a single dictionary installed in firefox.
A solution is to install alternative version, not shown by the addons page:
https://addons.mozilla.org/en-GB/firefox/addon/english-us-dictionary/versions/
There is a version that supports my firefox.
If this was helpful for you - please, leave a comment.