Many web sites use raw images of mathematical formulas generated from LaTeX. Fortunately, the source is generally available in the alt text so it is possible to convert them to MathML using the LaTeXML Web server. If you are concerned about speed & privacy, you can set up your own local server and modify the preference in about:addons. Note that by default the MathML output is automatically cached to speed up further processing. Finally, any MathML-aware accessible tool for Firefox will be able to read the mathematics.
Currently, only a few predefined rules are set, inluding *.wikipedia.org for Wikipedia web sites. There is an experimental user interface to add your own rules and you are encouraged to report them to the GitHub tracker.
See also the Native MathML add-on.
Bug Tracker: https://github.com/fred-wang/Mathzilla/issues