I’ve spent some time finding out a good french dictionary online, to get detailed definition, plurials, to check verbs usage. Here are my favorite tools. None are perfect, but they are free. I will update this post if any changes.
Lexilogos | **** it is a non commercial website dedicated to the french language and enabling you to search for a word definition in french amongst 8 different free online dictionnaries. By default, Trésor dictionnary will be used, which is fine as it is quite complete with 100.000 words and definitions.
“Mediadico familial” | *** a free dictionnary resource as well, for personal usage. I found the dictionnary a bit less precise and complete than Lexilogos. Moreover, you need to register to use the services.