Features:

Shows phonetic information
Shows part of speech
Hear pronunciation
Save a word to build your vocabulary
If a word is not found in the dictionary, it tries to show information from Google's featured search result.
Dark and light theme modes.

Usage:

Double-click a word to open the dictionary popup and view its meaning.
Or right-click on a selected word to open popup from menu.
View other definitions of the word using the 'More' link.
Hear the pronunciation using the speaker icon.
Save a word using the 'Save' button on the popup.
View all your saved words using the Dictionary Plus icon on your browser's toolbar.

Note: After installing, open another webpage to test the extension because Firefox doesn't allow running extensions on this website. Feedback: