This happens if you have deleted the profile folder or this profile got missing otherwise and the folder marked in profiles.ini as default=1 is no longer present.

• https://support.mozilla.org/en-US/kb/how-run-firefox-when-profile-missing-inaccessible

it seems like i have all my profiles when i look at the profile manager and the path of the default profile seems to match, but when the program tries to open it it still gives the error.

a weird thing is that firefox can open itself just fine but when the other program tries to open it it gets the error. is there something the program could be doing wrong?

i just tested it on Discord and discord seems to be able to open it fine, but this specific program give the error for firefox

it's probably a problem with the program itself

It would be interesting to know what Steam is actually sending Windows. As a possible roundabout way to discover that:

Open the Windows Task Manager (Ctrl+Shift+Esc), show More Details if needed, and switch to the Details tab.

Add the Command Line column (right-click any column to bring up a list of columns available). Then click the Command Line column heading to sort using that column, and scroll down to the Firefox lines. Let your eyes adjust for a moment... Typically the command lines looks like the one of these:

• Original startup process in default profile: "C:\Program Files\Mozilla Firefox\firefox.exe"
• External link/URL shortcut: "C:\Program Files\Mozilla Firefox\firefox.exe" -osint -url https://example.com
• Child process launched by Firefox: "C:\Program Files\Mozilla Firefox\firefox.exe" -content-proc [many parameters]

Keeping the Task Manager open, switch over to Steam to trigger the error.

Then after the error message appears, don't close it, but switch back to the Task Manager and see whether you can spot anything unusual about the new command line, for example, not being the same program path, or using an unusual command-line parameter.