A fix for this was made in pull request #129 upstream.
Fixed in vendordrop r32760
Note that these buttons do not work in VNC, so we'll have to test with rdesktop outside of ThinLinc.
Forward/back buttons work just fine through rdesktop. I was able to go back and forward through the history of my browser by using the mouse buttons.
Tested with an rdesktop from ThinLinc 4.8.0post_5572.