The addon is now archived (no more updates).
If it breaks in the future try the following alternative addon:
External Application Button
(by Andy Portmen)

I heard that the "Open With" developer no longer maintains the extension (I hope that some developer continues to update and maintain the extension, as it is very useful to open YT-DLP and video players such as MPV, PotPlayer, VLC and MPC- HC)

Open With is one of the few remaining Firefox extensions that are both extremely useful and actually working as designed. After downloading, follow the steps in the Open With Options page to install it.

First, use the "Fetch it from here" link to download the latest version of the Python software. Click on the downloaded installer to install Python. By default, it will install to C:\Users\[username]\AppData\Local\Programs.

Second, use the "Click here to download" link to download the Python file opem_with_windows.py. Move the file from the Downloads folder to the folder where you want it to be installed.

Third, use Command Prompt to install the .py file. It's a 2-step process. First, you have to tell Command Prompt where you put the file. Then you have to tell Command Prompt to install it.

For example, if you moved the .py file to C:\Users\[username]\Firefox, then the first command you would type in Command Prompt would be

cd "C:\Users\[username]\Firefox"

Press Enter. Then type

open_with_windows.py install

Press enter. Close Command Prompt.

Now, in the Open With Options page, click the "Test Installation" button. If a green check mark appears, you're done. If a red "X" appears, repeat the steps above until you get it right.

IMPORTANT NOTE: Understand that the Python software you downloaded enables the Python .py file to function, and the .py file enables the Open With extension to work. If you rename or move the folder in which you installed the .py file, the extension will stop working. In that case, use Command Prompt as above to reinstall the .py file in the new or renamed folder location.

This extension is absolutely amazing. I consider it one of the very select "Absolutely Mandatory and Indispensable" extensions I always use. I truly do not know how I lived without it, before first finding it. I don't know why Mozilla isn't pushing this as one of the *Recommended* extensions, because it should. Maybe I'm overestimating the number of people who would be interested in launching a webpage from one browser directly into a specific other browser, but for those that ever do this more than occasionally, this thing is terrific. I love Open With so much.

A little messy to install, but very useful when you get it going.

Works perfectly on Manjaro Linux! The AMO page should mention that the extension requires additional setup after installation. It took me a while to figure out that I need to go to the Options page after installing the extension.

A little touchy to install it for a newbie like me...
But once you follow the instructions given, it work like a charm!
Thanks a lot from a Waterfox Classic user.

AWESOME! I wanted such an extension for a long time! The best part? It supports Linux, unlike some other similar extensions! :D
This extension has a HUGE potential! I just added SMPlayer to watch YT vids in it and damn - it works! :D :D

Useful to run commands like mpv or yt-dlp. I've quickly reviewed parts of the source code, looks fine.

For the above commands, best practice is to add "--" as the last argument. (Maybe not a real concern, because the URL is passed as a single argument and probably cannot start with "-", but just in case.)

Thank you for a very nice add-on.
I have one request, but I want you to be able to export the settings

La verdad es fácil la instalación y funciona correctamente. sería bueno que estuviera para los demás exploradores.

Excellent add-on. Installation requires a few extra steps, but the instructions are very clear. Deserves donations.

I kept checking back to the reviews in hopes of more info on the installation of "open with", and there it was. I put open_with_windows.py in my Windows\system32......opened a command prompt, which opened in the Windows\system32 folder, and ran the open_with_windows.py install, and that was it for me. I ran test installation in my Browser and Thunderbird, and it showed, "Found version 7.2.3 at C:\​Windows\​System32\​open_with_windows.py." I then edited the order of my four Browsers, with Firefox on top.
Thanks to all the reviews with the help, and thanks to the the Author of this addon!!

Amazing addon! I'm only missing the ability to select multiple tabs and open them on the chosen browser, instead of doing it manually for each tab! There's an open issue on the github repo asking for this already, just in case other users would like to show their interest on this feature.