This is a very specific question most probably only German based users can help me with.
I registered with elster.de and finally got all info from them (the postal service took more than 2 months to deliver the needed letter) but login would not work.
I looked up the requirements and used a browser their site claims would work (a Firefox ESR) - but it did not.

I emailed their "technical hotline" and told them about it and that I did all their "technical requirements" said is needed for elster to work… and realized the folks answering questions have no clue about IT and especially about technical issues concerning Linux, they just kept repeating the same text as read from a fixed script… telling me info I already knew and being zero helpful about the issue…

Therefore: Who managed to get elster.de to run, and which browser did you use?
Or did you have to tweak settings, e.g. change what the browser tells the server what program and/or version it is?

Update
The alternative of using https://www.grundsteuererklaerung-fu...ateigentum.de/ is not possible for the Federal States of Germany I am living in. (as in: https://en.wikipedia.org/wiki/States_of_Germany )

This may not help much, as I do not speak or read German, but I was able to visit elster.de and navigate around the site with Firefox v. 106.0 (64-bit) on Slackware --Current. I was also able to do that in Konqueror v. 22.08.2.

I did not attempt to create an account or anything of that nature for obvious reasons, but the site seemed to be responsive.

Hope this helps at least a little.

Thanks frankbell, my FFx ESR was of an lower version number.
I try at least your FFx (non ESR) version and see how that goes.

(Just browsing works with my FFx ESR and with my standard Palemoon (newest x86-64 version) just fine. I do not get why they code a site so badly that browsers get trouble, it is a site paid via German tax money, they should be able to not code a crappy unreliable site. *growl*)

Hello/Hallo Rava,
I am running Slackware 15.0 with the Firefox browser (Help->About says 91.5.1esr (64-bit)) which I haven't updated yet. For Login I use a digital certificate and PIN and logging in works just fine.

Just tested it recently for the "Grundsteuererklärung" but eventually I found the ELSTER-form to be too complicated so I used a third-party service to submit the data for my 34m² of the Bundesrepublik Deutschland.