I found this
/usr/bin/browser --engine=microb
should work, regardless of the .browser configuration.

It didn't work.

In the strace, I did notice:
user:~$ strace /usr/bin/browser --engine=microb
execve("/usr/bin/browser", ["/usr/bin/browser", "--engine=microb"], [/* 61 vars */]) = 0
uname({sys="Linux", node="Nokia-N800-26", ...}) = 0
brk(0) = 0x12000
access("/etc/ld.so.preload", R_OK) = -1 ENOENT (No such file or directory)
open("/etc/ld.so.cache", O_RDONLY) = 3

Maybe there is a problem with the /etc/ld.so.conf file? Mine looks like this:

/usr/X11R6/lib
/usr/lib/minimo
/usr/local/xrdp
/usr/local/lib
/usr/lib
/lib
/usr/lib/microb-engine
/usr/lib/microb-engine/components