So you're saying all of the following is true: The default browser is set to Fennec (or anything other than MicroB, really).
Immediately after booting the device, no MicroB "browser" processes are seen.
~ $ pidof browser ~ $ ps|grep browser 1186 user 37980 S /usr/sbin/browserd -d 1534 user 2092 S grep browser
Immediately after booting the device and before opening MicroB for the first time, links in applications open in the chosen default browser (not MicroB).
~ $ pidof browser ~ $ ps|grep browser 1186 user 40232 S /usr/sbin/browserd -d 1713 user 3336 S /usr/bin/browser-switchboard 1803 user 60132 S /usr/sbin/browserd -s 1803 -n RTComMessagingServer
~ $ pidof browser 1853 1852 ~ $ ps|grep browser 1186 user 40232 S /usr/sbin/browserd -d 1713 user 3336 S /usr/bin/browser-switchboard 1803 user 60132 S /usr/sbin/browserd -s 1803 -n RTComMessagingServer 1851 user 40232 S /usr/sbin/browserd -d -b 1852 user 3936 S browser 1853 user 39312 S browser 1854 user 71336 S /usr/sbin/browserd -s 1854 -n browserui 1880 user 2092 S grep browser
MicroB opens when you select the Web entry in the menu. Two "browser" processes are seen.
When you close MicroB, the two "browser" processes remain running.
~ $ pidof browser 1853 1852 ~ $ ps|grep browser 1186 user 40232 S /usr/sbin/browserd -d 1713 user 3336 S /usr/bin/browser-switchboard 1803 user 60132 S /usr/sbin/browserd -s 1803 -n RTComMessagingServer 1851 user 40232 S /usr/sbin/browserd -d -b 1852 user 3936 S browser 1853 user 30640 S browser 1887 user 55716 S /usr/sbin/browserd -s 1887 -n browserui 1894 user 2092 S grep browser
After having opened and closed MicroB, links in applications open in MicroB, not the selected default browser. (This is a bug in Browser Switchboard, if true)
Did I understand you correctly?