Is the libXi (or possibly libinput) extension loaded?
You can get rid of several of the other warnings by loading the adwaita-icon-theme extension.
After some testing, I found out that:
1. "adwaita-icon-theme" is unnecessary for the toolbar buttons to work, it's not a part of the dep of Xorg-7.7
2. It doesn't look like a case of missing dependencies
3. Running firefox under Xorg-7.7 also gets the same warning
### boot code "base"
tce-load -i aterm wbar Xlibs Xfbdev Xprogs firefox flwm_topside
### buttons unclickable ...
tce-load -i Xorg-7.7
### with Xfbdev still running, unclickable ...
kill `cat /tmp/.X0-lock`
### buttons become functional while the terminal output remains the same.
It may have something to do with gtk3
firefox-ESR (45.6.0) has the dep with gtk2 and the toolbar works just fine.
However, running firefox (51.0.1) with both gtk2 and gtk3 under Xfbdev doesn't help.