This is a Polish Spellchecker Dictionary from https://addons.mozilla.org/en-US/firefox/addon/polish-spellchecker-dictionary/ combined with English US Dictionary from https://addons.mozilla.org/en-US/firefox/addon/us-english-dictionary/ It is created so you can type in both languages without changing any settings (currently the Firefox does not support multiple dictionaries).
To use paste included .xpi
file to extensions
dir in your Firefox profile dir. On windows it's:
C:\Users\user\AppData\Roaming\Mozilla\Firefox\Profiles\*.default\extensions
and select it under right-click menu (while typing).
Alternatively you can use it by building your own extension or replacing .dic
and .aff
files in existing one.