On LXQT at least the firefox file save dialog has a bug: if you try to change the name of the file to save it as, a search box pops up, instead of being able to edit the filename in the filename box. The filename box cannot be edited. Maybe the wrong xdg portal file dialog is used for file saving. Happens when right click to save images etc. Thank you for help with this.
It sounds like it might be specific to the LXQt frontend for the portal. Can you please file an issue upstream, and share the link to it here?
Also reported on Launchpad.