I don't have any specific information about the site or how it works, but I can provide general suggestions:

If a site is generally known to work in Firefox, these are standard suggestions to try when it stops working normally:

Double-check content blockers: Firefox's Tracking Protection feature, and extensions that counter ads and tracking, may break websites that embed third party content (meaning, from a secondary server).

(A) The shield icon toward the left end of the address bar usually turns a bit purplish when content is blocked. Click the icon to learn more or make an exception. See: Enhanced Tracking Protection in Firefox for desktop.

(B) Extensions such as Adblock Plus, Blur, Disconnect, DuckDuckGo Privacy Essentials, Ghostery, NoScript, Privacy Badger, uBlock Origin or uMatrix may block required elements on a site. Usually you can find a toolbar button for each add-on to manage blocked content in a page. There may or may not be a number on the icon indicating the number of blocked items; you sometimes need to click the button to see what's going on and test whether you need to make an exception for this site.

Cache and Cookies: When you have a problem with one particular site, a good "first thing to try" is clearing your Firefox cache and deleting your saved cookies for the site.

(1) Clear Firefox's Cache

See: How to clear the Firefox cache (only select Cached web content, don't clear all cookies and site data)

If you have a large hard drive, this might take a few minutes.

(2) Remove the site's cookies (save any pending work first). While viewing a page on the site, click the lock icon at the left end of the address bar. After a moment, a "Clear Cookies and Site Data" button should appear at the bottom. Go ahead and click that. Firefox will ask you to confirm; go ahead and confirm.

Then try reloading the page. Does that help?

Testing in Firefox's Troubleshoot Mode: In this mode, Firefox temporarily deactivates extensions, hardware acceleration, any userChrome.css/userContent.css files, and some other advanced features to help you assess whether these are causing the problem.

If Firefox is running:

You can restart Firefox in Troubleshoot Mode using either:

• "3-bar" menu button > Help > Troubleshoot Mode... (before Fx88: Restart with Add-ons Disabled)
• (menu bar) Help menu > Troubleshoot Mode... (before Fx88: Restart with Add-ons Disabled)

and OK the restart. A small dialog should appear. Click the Open button (before Fx88: "Start in Safe Mode" button).

If Firefox is not running:

Hold down the Shift key when starting Firefox. (On Mac, hold down the option/alt key instead of the Shift key.) A small dialog should appear. Click the Open button (before Fx88: "Start in Safe Mode" button).

Note: Don't use the Refresh without first reviewing this article to understand what will be deleted: Refresh Firefox - reset add-ons and settings.

Any improvement?

Hello tried all suggestions, but no success.

I get different results when I click "Accesso a mi DGT".

Regular Profile

With my various custom settings and blockers, the site forwards to https://pasarela.clave.gob.es/Proxy2/ServiceProvider and displays:

Se ha producido un error... invalid.sp.domain Por favor, reinténtelo de nuevo más tarde

Test Profile

In a separate profile, after the progress wheel goes around for a while, that page loads and asks me for a Cl@ve PIN, etc. If you want to create a test profile, I'll describe the steps below the line.

I don't get an XML error up to that point. Does the problem arise after another step?

New Profile Test

This takes about 3 minutes, plus the time to test your problem site(s).

Inside Firefox, type or paste about:profiles in the address bar and press Enter/Return to load it.

Take a quick glance at the page and make a mental note of which Profile has this notation: This is the profile in use and it cannot be deleted. That is your current default profile.

Click the "Create a New Profile" button, then click Next. Assign a name like Test2023, ignore the option to relocate the profile folder, and click the Finish button.

Firefox will switch your default profile to the new one, so click the Set as Default Profile button for your regular one to avoid an unwanted surprise at your next startup.

Scroll down to Test2023 and click its Launch profile in new browser button.

Firefox should open a new window that looks like a brand new, uncustomized installation. (Your existing Firefox window(s) should not be affected.) Please ignore any tabs enticing you to connect to a Sync account or to activate extensions found on your system to get a clean test.

Do the problem site(s) work any better in the new profile?

When you are done with the experiment, you can close the extra window without affecting your regular Firefox profile. (Test2023 will remain available for future testing.)

Hello First, many thanks for your suggestions and effort to help. I have now tried your suggestion using a test profile in firefox as you have explained.

I get the exact same result. Below I will give a more detailed sequence leading up to the message: 1. https://sede.dgt.gob.es/es/

   press: Acceso a mi DGT

 Then I get 3 choices (step 2 below) to go further: 2 Cl@ve choices which is irrelevant to me(I have no Cl@ve)
 I choose  'eldentifier'  which I think is to use my obtained certificate.

2. https://sede.dgt.gob.es/es/mi_dgt/index.shtml

    press:eldentifier

Then I am forwarded to step 3.

3. https://pasarela-ident.clave.gob.es/IdP2/AuthenticateCitizen

    (Shows a few second with a message I am not able to catch before step 4 below )

4. https://sede.dgt.gob.es/es/mi_dgt/index.shtml

Giving the below message:

XML Parsing Error: no root element found Location: https://sede.dgt.gob.es/es/mi_dgt/index.shtml Line Number 1, Column 1:

Hope this can help to find where the problem is.