I just wanted to browse 3rd party MIBs that weren't installed in the system-wide MIB location and seemingly, mbrowse supports only one additional MIB via the "Open MIB" dialog (please correct me if I am wrong). With the MIBs in question this was a problem as they had inter-dependencies and I needed multiple of them to be loaded at the same time.
As a workaround, I used the environment variables which are parsed by the netsnmp libs (MIBDIRS=+/path/to/additional/mibs MIBS=ALL), but it would be helpful for beginners if mbrowse would have a GUI-way to load multiple additional MIBS (and maybe a list of loaded MIBS).
Addendum: Tested with 0.31, my distribution doesn't yet have 0.40 available. If this feature is available in 0.40 please let me know (ChangeLog didn't suggest it) and disregard this request.
adding from tracker item 3103789: the (planned) GUI for loading MIBs should remember the list of loaded MIBs across restarts