This is a long time problem, like 9 years ago one of the first ticket was opened to bugzilla about get the KDE filepicker support in Firefox.
Finally, with Firefox 64 this is possible!
Looking at the release post of Plasma 5.15 that explain how to do it.
In my case in Debian Sid to get the support I need to have Firefox 64 or more new version. In my case is easy because I use the Nightly version and Developer edition (beta) so I am already covered.
Next is required to install 2 packages:
apt install xdg-desktop-portal xdg-desktop-portal-kde
The last one is to change the
.desktop file to add a new variable
In my case this is not possible because I am using the Firefox build from Mozilla that not include them, so I can achieve the same result with:
Another solution is to add this line in your
That’s it! Enjoy the native experience in Firefox!