Useful extension but unfortunately it requires few extra steps maybe not easy to understand by basic users.

Donation made. Thank you 👍

If you are on macOS Catalina, install this extension and follow the instructions, but put the file "open_with_mac.py" in your "Public" folder. Only then it will work.

For people who cannot use it on macOS Catalina. I found a solution here, https://stackoverflow.com/questions/59543692/failed-to-connect-native-host-has-exited . In other words, download the helper program open_with_mac.py to $HOME/Public .
For debugging, I find it easier to debug with Chrome browser. There is a Chrome version of this extension https://darktrojan.github.io/openwith/

I was going to give 1 star. Till I realized I was in older python folder on main C Directory was wrong, I removed older versions and Launcher of Python from control panel (Programs and Features). and reloaded Python version 3.8.1 and noticed where it was loading it (Windows) >
C:\Users\xxx\AppData\Local\Programs\Python
I dumped the "open_with_windows.py" File downloaded from (On the Option Page when you click the toolbar button) I placed that file after downloading it in the Python folder and then Ran the "open_with_windows.py install" in Command Prompt Admin THEN Tested it via > Options on the Open With toolbar button got a green check and then it worked when I tried it on a web site page.

Finally, the installation is successful,Set the default open mode to Python,and use "open_with_windows.py install"
in the folder where the file is located. If you open any other windows when use "open_with_windows.py install" ,you should set the default open mode of .py file. Chinglish

Excellent.

Installed the add-on & helper on my Linux system, and found it very easy to manually add a lightweight browser (Qupzilla) to the list.

Not easy to install. tried doing this on my FF Portable. Installed the Python program and ran the command "open_with_windows.py install" Nothing happens and the open_with_windows.py file is opend in my text editor. When I click the Test Installation button get a cryptic message"Something went wrong. There might be more information in the Browser Console."

Extremely poor documentation. Don't know whether this works with FF Portable.

Thanks for the Plugin! Good but complicated. Have written a small tutorial in english and german. Maybe it helps someone.

DE: Gut aber kompliziert zu installieren. Wer Hilfe braucht kann es mit der kleinen Anleitung versuchen.

https://plugins.onkel-franky.de/tests/other-stuff/tutorial-firefox-plugin-open-with

Open With saves me a whole lot of hassle. Some sites don't work properly in FF, so, using Open With I can quickly load the current page in Chrome or (perish the thought!) Edge. But still more useful and indeed important for me is that I can quickly load the current page in BlueGriffon, my web page editor, or in EditPad Pro. I use both these latter for working on my local version of my extensive five websites. Saves a huge amount of hassle!

One odd thing. Just lately, the toolbar icon for Open With has taken to repeatedly giving a warning message, that Open With couldn't make contact with the outside world (presumably meaning it couldn't make an Internet connection). But its permissions don't include being able to do so, so something is clearly screwy about this.

As another reviewer has remarked, it would be greatly helpful to have a facility for exporting / importing one's settings.

Update:
I found that this extension had ceased working, and presumably this was associated with the error message noted above. I followed a reasonable hunch and re-ran "[path]open_with_windows.py install", and that fixed it.

I would not be able to live without this extension! Thanks Geoff for putting in all the work into it. I have a handful of privacy friendly extensions installed like Umatrix and ublock origin and a couple of other othes that often times break websites that i'm visiting for the first time. Most of the time i don't want to bother customizing the rules for the website because i probably will not be visiting it again so i quickly open the page using a different firefox profile and continue with my workflow. Makes my life incredibly easy and am very glad to have donated the $10 to try to pay back for all the time that i have saved using this.

great.

please, create option to export/import all configuration.

Please, update the extension to the latest version of Firefox!

FF60.6 ESR x64 - I always used this extension to watch YT videos in Firefox (browser-independent, better volume control). It worked perfectly, 5 stars.
For the new Firefox now it needs some extra steps, but it's nothing too difficult.
The problem, however, is that here CMD.exe for security resons is only allowed to run with admin permissions, so this addon it's KO for me unless it will manage to use other methods :\