The setting okular as a default viewer for the PDF in the Iceweasel Preferences
window is ignored.

Steps to reproduce:
1) open the Application preferences (Edit|Preferences|Applications)
2) in the list of content types look-up the PDF
3) change setting to use okular instead of previewing the document in the
Iceweasel
4) Close the window using OK button.
5) Restart iceweasel

Expected:
The PDF document shall be opened in the external viewer (okular).

Actual:
The PDF document is still opened in the internal PDFJS viewer.

Even after disabling the PDFJS using the option pdfjs.disabled in the
about:config is a PDF document opened in the internal viewer.

src:iceweasel has been superseded by src:firefox-esr in version
45.0esr-1 in March 2016. Transitional packages to ease upgrades were
provided in the wheezy, jessie, stretch and buster releases. The
transitional packages have been removed finally before the bullseye
release in August 2021.
After regular security support for buster ended in August 2022 and LTS
support ended in June 2024, I'm closing the remaining bug reports now.

Andreas