It may be odd, but bookmarklets are the quickest way to 'mod' a browser session, and a welcome leftover from a simpler time (for now even working on some mobile browsers).

For instance, I have one setup that after activation, allows me to highlight text on a page, which is then automaticaaly dated, annotated with metadata and copied to my clipboard, saving a lot of keystrokes and touch movements. Another parses the .bib metadata found in Google Scholar results, and prettyfies it to a markdown suitable format.

I would dread losing these simple scripts that can be run across all my devices.