<s>
Firefox	B-Language
User	I-Language
Extension	I-Language
Library	I-Language
(	O
FUEL	O
)	O
was	O
a	O
JavaScript	B-Language
library	I-Language
intended	O
for	O
developing	O
Mozilla	B-Application
Firefox	I-Application
extensions	B-Operating_System
.	O
</s>
<s>
Co-created	O
by	O
Mark	O
Finkle	O
and	O
John	O
Resig	O
,	O
it	O
provided	O
JavaScript	B-Language
libraries	I-Language
and	O
wrappers	B-Library
for	O
the	O
most	O
commonly-used	O
operations	O
in	O
Firefox	B-Operating_System
extensions	I-Operating_System
.	O
</s>
<s>
FUEL	O
was	O
intended	O
to	O
narrow	O
the	O
gap	O
between	O
two	O
modes	O
of	O
development	O
in	O
Firefox	B-Application
.	O
</s>
<s>
One	O
mode	O
of	O
development	O
extends	O
from	O
the	O
core	O
elements	O
of	O
Firefox	B-Application
and	O
the	O
Firefox	B-Application
code	O
base	O
,	O
which	O
is	O
predominantly	O
based	O
on	O
C++	B-Language
.	O
</s>
<s>
The	O
other	O
mode	O
of	O
development	O
extends	O
from	O
the	O
client-side	O
functionality	O
of	O
the	O
browser	O
itself	O
,	O
which	O
is	O
predominantly	O
based	O
on	O
JavaScript	B-Language
.	O
</s>
<s>
FUEL	O
was	O
intended	O
to	O
make	O
extensions	B-Operating_System
easier	O
to	O
create	O
,	O
especially	O
for	O
this	O
latter	O
group	O
of	O
people	O
.	O
</s>
<s>
FUEL	O
was	O
marked	O
as	O
deprecated	O
in	O
Firefox	B-Application
40	O
and	O
was	O
removed	O
in	O
Firefox	B-Application
47	O
.	O
</s>
