Web Compatibility Support • Re: www.bahn.de/buchung isn't working—or is it?

The general.useragent.compatMode.firefox setting was set to “false” on the Palemoon that was working, and to “true” on the other computer (as is default). Unfortunately, I can't verify this, because for some reason, it seems that changes no longer can be made to this particular entry — neither double-clicking nor using the context menu works to change the value.

That configuration is set from the Preferences menu. Tools - Preferences - Advanced - General tab - set User Agent Mode to "Native", "Gecko Compatibility", or "Firefox Compatibility". You can watch the about:config preference change in real time as you make the change in the Preferences menu.