<s>
XUL	B-Application
(	O
)	O
,	O
which	O
stands	O
for	O
XML	B-Application
User	I-Application
Interface	I-Application
Language	I-Application
,	O
is	O
a	O
user	B-Language
interface	I-Language
markup	I-Language
language	I-Language
developed	O
by	O
Mozilla	B-Operating_System
.	O
</s>
<s>
XUL	B-Application
is	O
an	O
XML	B-Protocol
dialect	O
for	O
writing	O
graphical	B-Application
user	I-Application
interfaces	I-Application
,	O
enabling	O
developers	O
to	O
write	O
user	B-Application
interface	I-Application
elements	O
in	O
a	O
manner	O
similar	O
to	O
web	O
pages	O
.	O
</s>
<s>
XUL	B-Application
applications	B-Application
rely	O
on	O
the	O
Mozilla	B-Operating_System
codebase	B-General_Concept
or	O
a	O
fork	B-Application
of	O
it	O
.	O
</s>
<s>
The	O
most	O
prominent	O
example	O
is	O
the	O
Firefox	B-Application
web	I-Application
browser	I-Application
.	O
</s>
<s>
However	O
,	O
Mozilla	B-Operating_System
has	O
reduced	O
the	O
usage	O
of	O
XUL	B-Application
in	O
Firefox	B-Application
after	O
their	O
rewrite	O
of	O
the	O
browser	B-Protocol
engine	I-Protocol
in	O
version	O
57	O
.	O
</s>
<s>
This	O
resulted	O
in	O
the	O
removal	O
of	O
add-on	B-Operating_System
customization	O
.	O
</s>
<s>
Firefox	B-Application
originally	O
permitted	O
add-ons	B-Operating_System
to	O
extensively	O
alter	O
its	O
user	B-Application
interface	I-Application
,	O
but	O
this	O
capability	O
was	O
removed	O
in	O
2017	O
and	O
replaced	O
with	O
the	O
less-permissive	O
WebExtensions	B-Operating_System
API	O
.	O
</s>
<s>
Several	O
forks	B-Application
of	O
Firefox	B-Application
retain	O
support	O
for	O
XUL	B-Application
and	O
XUL-based	O
add-ons	B-Operating_System
.	O
</s>
<s>
Waterfox	B-Language
maintains	O
a	O
fork	B-Application
of	O
the	O
Mozilla	B-Operating_System
codebase	B-General_Concept
for	O
the	O
Waterfox	B-Language
Classic	O
browser	O
.	O
</s>
<s>
The	O
Pale	B-Protocol
Moon	I-Protocol
browser	I-Protocol
,	O
Basilisk	O
browser	O
,	O
Hyperbola	O
operating	O
system	O
,	O
and	O
several	O
smaller	O
projects	O
collectively	O
use	O
and	O
maintain	O
the	O
Unified	O
XUL	B-Application
Platform	O
(	O
UXP	O
)	O
implementation	O
.	O
</s>
<s>
XUL	B-Application
was	O
devised	O
at	O
Netscape	B-Protocol
in	O
1997	O
as	O
part	O
of	O
the	O
development	O
effort	O
that	O
eventually	O
became	O
the	O
Mozilla	B-Operating_System
codebase	B-General_Concept
.	O
</s>
<s>
XUL	B-Application
is	O
similar	O
to	O
web	O
technologies	O
implemented	O
by	O
the	O
Gecko	B-Language
rendering	I-Language
engine	I-Language
.	O
</s>
<s>
When	O
XUL	B-Application
was	O
introduced	O
,	O
it	O
added	O
features	O
beyond	O
the	O
then	O
standard	O
HTML	B-Language
4	I-Language
.	O
</s>
<s>
It	O
was	O
designed	O
as	O
an	O
expansion	O
of	O
HTML	B-Language
to	O
be	O
used	O
with	O
applications	B-Application
rather	O
than	O
documents	O
.	O
</s>
<s>
XUL	B-Application
was	O
used	O
by	O
various	O
Mozilla	B-Operating_System
projects	I-Operating_System
,	O
forks	B-Application
of	O
Mozilla	B-Operating_System
projects	I-Operating_System
,	O
and	O
projects	O
closely	O
related	O
to	O
Mozilla	B-Operating_System
.	O
</s>
<s>
Some	O
software	O
projects	O
like	O
Zotero	B-Application
began	O
as	O
Mozilla	B-Application
Firefox	I-Application
extensions	B-Protocol
and	O
used	O
XULRunner	B-Language
to	O
become	O
standalone	O
desktop	B-Application
applications	I-Application
.	O
</s>
<s>
In	O
the	O
early	O
2000s	O
there	O
was	O
some	O
interest	O
in	O
using	O
XUL	B-Application
by	O
other	O
parties	O
,	O
including	O
Amazon	B-Application
,	O
but	O
those	O
have	O
switched	O
over	O
to	O
HTML5	B-Application
.	O
</s>
<s>
The	O
features	O
of	O
HTML5	B-Application
that	O
made	O
web	O
applications	B-Application
possible	O
,	O
also	O
made	O
much	O
of	O
XUL	B-Application
redundant	O
.	O
</s>
<s>
Mozilla	B-Operating_System
released	O
Firefox	B-Application
57	O
,	O
also	O
known	O
as	O
Firefox	B-Application
Quantum	O
,	O
in	O
2017	O
.	O
</s>
<s>
As	O
part	O
of	O
this	O
,	O
Firefox	B-Application
finished	O
transitioning	O
to	O
a	O
multi-process	O
architecture	O
.	O
</s>
<s>
Although	O
XUL	B-Application
add-ons	B-Operating_System
worked	O
in	O
this	O
architecture	O
,	O
they	O
created	O
large	O
performance	O
problems	O
.	O
</s>
<s>
Because	O
of	O
this	O
,	O
Mozilla	B-Operating_System
removed	O
support	O
for	O
legacy	O
add-ons	B-Operating_System
,	O
including	O
the	O
use	O
of	O
custom	O
XUL	B-Application
code	O
.	O
</s>
<s>
Mozilla	B-Operating_System
switched	O
extensions	B-Protocol
over	O
to	O
Google	O
's	O
HTML5-based	O
WebExtensions	B-Operating_System
format	O
.	O
</s>
<s>
In	O
2017	O
,	O
Firefox	B-Application
still	O
included	O
289	O
XUL	B-Application
bindings	O
for	O
native	O
browser	O
components	O
.	O
</s>
<s>
By	O
the	O
end	O
of	O
2019	O
,	O
Mozilla	B-Operating_System
had	O
removed	O
all	O
XUL	B-Application
files	O
from	O
their	O
mozilla-central	O
codebase	B-General_Concept
.	O
</s>
<s>
When	O
Mozilla	B-Operating_System
removed	O
support	O
from	O
Firefox	B-Application
,	O
there	O
were	O
still	O
several	O
applications	B-Application
using	O
XUL	B-Application
.	O
</s>
<s>
Mozilla	B-Operating_System
began	O
gradually	O
removing	O
XUL	B-Application
support	O
from	O
their	O
email	O
client	O
,	O
Thunderbird	B-Application
.	O
</s>
<s>
They	O
released	O
Thunderbird	B-Application
78	O
without	O
support	O
for	O
XUL-based	O
extensions	B-Protocol
in	O
2020	O
.	O
</s>
<s>
The	O
SeaMonkey	B-Language
internet	I-Language
suite	I-Language
chose	O
to	O
re-implement	O
XUL	B-Application
for	O
the	O
Firefox	B-Application
Quantum	O
codebase	B-General_Concept
and	O
the	O
suite	O
's	O
development	O
has	O
subsequently	O
slowed	O
.	O
</s>
<s>
The	O
Pale	B-Protocol
Moon	I-Protocol
developers	O
forked	O
Mozilla	B-Operating_System
's	O
entire	O
Firefox	B-Application
52	O
ESR	O
codebase	B-General_Concept
to	O
create	O
the	O
Unified	O
XUL	B-Application
Platform	O
(	O
UXP	O
)	O
for	O
their	O
Goanna	B-Language
rendering	O
engine	B-Protocol
.	O
</s>
<s>
UXP	O
maintains	O
the	O
traditional	O
XUL	B-Application
capabilities	O
.	O
</s>
<s>
As	O
a	O
result	O
,	O
Pale	B-Protocol
Moon	I-Protocol
and	O
other	O
UXP	O
applications	B-Application
have	O
remained	O
single-process	O
but	O
extensible	O
.	O
</s>
<s>
XUL	B-Application
can	O
only	O
be	O
used	O
with	O
the	O
Mozilla	B-Operating_System
codebase	B-General_Concept
(	O
or	O
a	O
fork	B-Application
of	O
it	O
)	O
because	O
the	O
Gecko	B-Language
engine	I-Language
does	O
the	O
XUL	B-Application
rendering	O
.	O
</s>
<s>
Application	O
programmers	O
need	O
to	O
define	O
a	O
XUL	B-Application
interface	O
as	O
three	O
discrete	O
sets	O
of	O
components	O
:	O
</s>
<s>
XUL	B-Application
defines	O
a	O
wide	O
range	O
of	O
elements	O
,	O
which	O
roughly	O
belong	O
to	O
the	O
following	O
types	O
:	O
</s>
<s>
The	O
default	O
behavior	O
of	O
XUL	B-Application
widgets	O
can	O
be	O
altered	O
with	O
XBL	B-Application
bindings	O
.	O
</s>
<s>
The	O
villain	O
of	O
the	O
1984	O
film	O
Ghostbusters	B-Operating_System
was	O
a	O
deity	O
called	O
Zuul	O
who	O
possesses	O
the	O
character	O
Dana	O
Barrett	O
and	O
declares	O
,	O
"	O
There	O
is	O
no	O
Dana	O
.	O
</s>
<s>
The	O
creators	O
of	O
XUL	B-Application
,	O
which	O
is	O
pronounced	O
the	O
same	O
as	O
Zuul	O
,	O
made	O
the	O
slogan	O
"	O
There	O
is	O
no	O
data	O
.	O
</s>
<s>
There	O
is	O
only	O
XUL	B-Application
!	O
</s>
<s>
"	O
,	O
part	O
of	O
which	O
became	O
the	O
XML	B-Language
namespace	I-Language
.	O
</s>
