The tools are there, if you care to use them.
A combination of very restrictive globals.local and <appname>.local files added to Firejail + custom apparmor rules allows me to jail applications as well or likely better than is possible on any other platform. Other tools are available that accomplish the same, if either or both of these is not to your liking (firejail has improved greatly with recent versions, if your opinion was formed years ago, but not all default profiles have migrated to restrictive explicit allow policies vs. permissive explicit deny policies yet, and those old policies are a pita to add your own restrictions to).
You could also just live with the default rule sets Firejail and Apparmor ship with, and have less restrictively jailed apps by default, but in many cases sufficiently jailed to prevent access to e.g., your ssh-agent without any additional effort-- in this case jailing is just install firejail+apparmor and create a few symlinks for the applications you want to be jailed (IMO still not a high effort bar; and, there is a single command that will create symlinks for you, for everything firejail ships a default profile for, if you feel comfortable running that).
Nothing that interacts with the Internet or untrusted files has access to anything confidential in my home dir, or anywhere else on my system unless that access is needed (including the named pipe for ssh-agent). None of these applications have network access that don't need it. Apps can only use syscalls that are explicitly listed, cannot gain capabilities beyond the restricted capabilities listed, only have access to the bins necessary to run, can only see existence of processes in their own jail, etc.
If, privileged filesystem or network access isn't required as the rule, I'll add a separate profile with extra privileges, and the default profile will not have that access. E.g., for pdf files, I have 'atril' and 'atril-privileged'. Neither atril profile has network access, nor access to ssh keys, global ~/.cache, etc. The day to day one can only read from a few locations on the filesystem, and can only write to two paths. The privileged one can also read where I save private info like tax information.
Firefox profiles run entirely in tmpfs, with each profile having its own ~/.cache ~/Downloads (the real ~/Downloads contains symlinks to each of the jailed Download dirs), etc. I setup the tmpfs for firefox and chromium with a wrapper script, so I can easily handle data that I want to persist and to work around limitations in Firejail. There are separate launchers (rarely used) for Firefox/chromium which allow persistently modifying the profiles (most firefox config is via user.js and misc sqlite/json files, but I'm unaware of an equivalent for chromium).
DBUS is a pita for jails, though. To prevent escaping your private mount namespace jail, you really have to kill dbus access inside the jail (which requires a private net namespace or disallowing unix domain sockets to be really sure; but sometimes all you can do is add a filter-- Firejail allows for all of these options [with limitations on net namespace support for unpriv users] ).
Another addition to my setup that made it so the above restrictions were more or less transparent is a script that runs as a daemon outside the jails and gets fed filenames and urls from another script that runs inside each jail that is symlinked as e.g., symlink name 'atril' for launching a pdf viewer from within a jail-- the outer daemon script will launch an new jail instance of atril with the file passed from e.g., the thunderbird jail by the helper script inside that jail. For URLs, the same jail instance of firefox is re-used.