So just as a caveat, I imagine Tridactyl would really mostly be appreciated by those with a modal, and specifically Vim inspired mentality; its mission, after all, is to bring vim-like bindings and workflow to Firefox. This is mostly to say, it may not appeal to you otherwise (but who knows!)

If you are already familiar with how key bindings are set in vim you'll hit the ground running. In fact, many keys are pretty intuitive since they match vim, eg, scrolling up/down is controlled with j or k.

I may not use every single function built into Tridactyl everyday, but as a person who likes to reduce his reliance on a mouse, I can easily navigate both a page and the web at large entirely with my keyboard. Typing f puts a hint at every link that you can follow by typing the letter in the hint. ]] or [[ can auto increment pages on forums (eg going from page 2 to page 3). I can quickly traverse my history, bookmarks, etc with a command prompt that can also access nearly every feature of Firefox. I often use a binding to pin tabs or close them, etc.

On a regular day that might be all I do.

On the other end of the spectrum, I'll give a more extreme example. A friend needed help with his company's wordpress site. They had a couple hundred articles that needed a uniform change. While there was probably an easier and smarter way of doing it, I used Tridactyl (with a healthy dose of pyAutoGui) to automate it. I made a couple of commands in Tridactyl to do things like open certain links as new tabs, navigate to each tab, open the WYSIWYG editor for each page, locate particular text, delete and replace it), save, and move to the next tab and repeat. I was able to do this with about 10-15 articles at a time...I got paid to press a couple keys, walk off to do something with my kid and come back to check on it from time to time (I added in fail-safes for when it needed manual intervention). Admittedly, this did go beyond the scope of Tridactyl, but it was an invaluable part of the whole deal.

Another time I was doing a data entry job and needed to transfer both the hyperlink of, and several pieces of info, into a spreadsheet. It occurred to me that it would be nice to grab the URLs of all the pages I had open at once instead of manually going to each tab copying the url, alt-tabbing to the spreadsheet and pasting just to alt tab back to FF going to a new tab copying the url and so on.

The creator of Tridactyl helped me write a command that allowed me to open as many tabs as necessary, and copy to the clipboard every URL of each tab open from the one I was on until there were no more tabs, each separated by a comma to easily paste into the spreadsheet. Saved me so much time and carpal tunnel.

Ultimately, describing a few things I've used it for is a disservice because if you ask the next person, they'll use it completely differently.