My i3status bar overlaps the firefox windows but not other windows (gvim), this affects the ability to view some data at the bottom of web pages.
My setup is Debian unstable, i3 version 4.15 and i3status version 2.12.
The bar is setup with a rather empty looking:
bar {
status_command i3status
}
I just ran into this myself on Debian Buster, with i3 4.16.1-1, i3status 2.12-1, and firefox-esr 60.8.0esr-1~deb10u1.
Try the following steps:
1) Switch the firefox window from tiled to floating
2) If the window is very large, resize it to something much smaller - leave several inches of space on each side.
3) Exit firefox
4) Start firefox
After the above steps, mine worked normally without overlapping the status bar.
If that doesn't work, you might want to try starting firefox with a new profile and see if that helps.