I would also like to know how and why this is installed in my browser... I literally just installed and was setting up Firefox that came with my Distribution. The only thing I did was change some settings and import my bookmarks! This is such BS!!!
It was great but it won't work anymore, hasn't worked for me for about 6 months. I keep hoping the next Firefox update will fix it (now on 62.03). With every update I remove it, add it again but it wont come up in the dictionaries list.