Multi-Keywords Highlighter от Iván Ruvalcaba

Multi-Keywords Highlighter is a browser extension for Firefox (version 52.0 or higher). It's intended to be assistive for searching for the interesting bits of a web page by defining a keyword list.

Once you have defined the keyword list, Multi-Keywords Highlighter shows them to you instantly through beautiful and colorful highlighters (ten different colors to be more precise). In addition, is also able to point out to you the number of matches on the current web page.

The keyword list is stored locally so you shouldn't worry about your privacy or anonymity.