This add-on allows to search in CNRTL (Centre national de ressources textuelles et lexicales) dictionnaries, while reading any page.
Available for Firefox here: https://addons.mozilla.org/firefox/addon/cnrtl-addon/
Choose your preferred dictionary on this add-on's setting page among Lexicography (definition), Synonymy, Antonymy, Etymology, Morphology, Proxemics and Consistency. Note that all of these are French dictionnaries.
When reading any web page, select (double click) the word you wanna search for, and open Firefox context menu (right click), then just choose CNRTL option and the entry in the chosen dictionary for the selected word will be opened in a new tab.