I have the same problem.
I am using Firefox ESR (on GNU/Linux Debian 11.3).
The "save as" dialog lists uppercase folders first, for example:
"Anything; Backup; aNewFolder; bb-8"
This Firefox "About" dialog shows "91.10.0esr (64-bits)" as version identifier.
I usualy keep it configured to use "Portuguese (Brazil)" as default language.
Changing the language to "English (United States)" does NOT solve the issue.
I have also tried running it, via terminal, with:
~$ LC_COLLATE=C LC_ALL=C firefox
But it does NOT solve the issue, either.
But, I also have Firefox "Stable" installed in my home folder.
I've downloaded the 'firefox-94.0.2.tar.bz2' package from Mozilla
website, sometime ago, and simply uncompressed it into '~/firefox/'.
When I tested this version, the "save as" dialog sorted the folders
and files as expected (mixing lowercase an uppercase) like:
aNewFolder; Anything; Backup; bb-8
This Firefox currently shows "101.0.1 (64-bit)" as version identifier.
It is configured with "English (US)" as default language.
I am using XFCE, and Thunar as default file manager.
I've already changed the hidden configuration 'misc-case-sensitive',
as described here 'https://docs.xfce.org/xfce/thunar/hidden-settings'.
But it does not seem to affect the browser's dialog.
AFAIK, any other instance of Thunar respects this setting.
In a quick search, the system's configured 'locale' seemed to
have some importance on the matter.
So here are some info about my system:
~$ locale -a
C
C.UTF-8
POSIX
pt_BR.utf8
And:
~$ cat /etc/default/locale
# File generated by update-locale
LANG="pt_BR.UTF-8"
LANGUAGE="pt_BR:pt:en"
