7 letter words:
firebug13,
6 letter words:
figure10,
5 letter words:
brief10,
fiber10,
fibre10,
grief9,
giber8,
4 letter words:
frig8,
frug8,
berg7,
brig7,
burg7,
fire7,
gibe7,
grub7,
reif7,
rife7,
bier6,
brie6,
rube6,
grue5,
urge5,
3 letter words:
fib8,
fub8,
fig7,
fug7,
beg6,
big6,
bug6,
fer6,
feu6,
fie6,
fir6,
fur6,
gib6,
ref6,
rif6,
bur5,
reb5,
rib5,
rub5,
urb5,
erg4,
gie4,
reg4,
rig4,
rug4,
ire3,
rei3,
rue3,
2 letter words:
ef5,
fe5,
if5,
be4,
bi4,
er2,
re2,
