<s>
Add-on	B-Operating_System
is	O
the	O
Mozilla	B-Operating_System
term	O
for	O
software	O
modules	O
that	O
can	O
be	O
added	O
to	O
the	O
Firefox	B-Application
web	I-Application
browser	I-Application
and	O
related	O
applications	B-Application
.	O
</s>
<s>
Mozilla	B-Operating_System
hosts	O
them	O
on	O
its	O
official	O
add-on	B-Operating_System
website	O
.	O
</s>
<s>
Browser	B-Protocol
extensions	I-Protocol
are	O
the	O
primary	O
type	O
of	O
add-on	B-Operating_System
.	O
</s>
<s>
In	O
2017	O
,	O
Mozilla	B-Operating_System
enacted	O
major	O
changes	O
to	O
the	O
application	B-Application
programming	I-Application
interface	I-Application
(	O
API	B-General_Concept
)	O
for	O
extensions	B-Protocol
in	O
Firefox	B-Application
,	O
replacing	O
the	O
long-standing	O
XUL	B-Application
and	O
XPCOM	B-Application
APIs	B-General_Concept
with	O
the	O
WebExtensions	B-Operating_System
API	B-General_Concept
that	O
is	O
modeled	O
after	O
Google	B-Application
Chrome	I-Application
's	O
API	B-General_Concept
.	O
</s>
<s>
Thus	O
add-ons	B-Operating_System
that	O
remain	O
compatible	O
with	O
Firefox	B-Application
are	O
now	O
largely	O
compatible	O
with	O
Chrome	B-Application
as	O
well	O
.	O
</s>
<s>
As	O
of	O
December	O
,	O
2022	O
,	O
there	O
are	O
close	O
to	O
30,000	O
add-ons	B-Operating_System
and	O
over	O
480,000	O
themes	B-Application
available	O
for	O
Firefox	B-Application
.	O
</s>
<s>
Starting	O
with	O
Firefox	B-Application
57	O
,	O
only	O
the	O
new	O
WebExtensions	B-Operating_System
API	B-General_Concept
is	O
supported	O
.	O
</s>
<s>
Early	O
versions	O
of	O
Firefox	B-Application
supported	O
themes	B-Application
that	O
could	O
greatly	O
change	O
the	O
appearance	O
of	O
the	O
browser	B-Application
,	O
but	O
this	O
was	O
scaled	O
back	O
over	O
time	O
.	O
</s>
<s>
Current	O
themes	B-Application
are	O
limited	O
to	O
changing	O
the	O
background	O
and	O
text	O
color	O
of	O
toolbars	O
.	O
</s>
<s>
(	O
These	O
lightweight	O
themes	B-Application
were	O
formerly	O
called	O
personas	O
.	O
)	O
</s>
<s>
Prior	O
to	O
2017	O
,	O
Firefox	B-Application
supported	O
extensions	B-Protocol
developed	O
with	O
different	O
APIs	B-General_Concept
:	O
XUL	B-Application
,	O
XPCOM	B-Application
,	O
and	O
Jetpack	B-Application
.	O
</s>
<s>
Mozilla	B-Operating_System
now	O
refers	O
to	O
these	O
as	O
legacy	O
extensions	B-Protocol
.	O
</s>
<s>
Plug-ins	B-Application
are	O
no	O
longer	O
supported	O
in	O
Firefox	B-Application
.	O
</s>
<s>
In	O
the	O
past	O
,	O
they	O
were	O
used	O
to	O
handle	O
media	B-Application
types	I-Application
for	O
which	O
the	O
application	O
did	O
not	O
have	O
built-in	O
capability	O
.	O
</s>
<s>
They	O
were	O
deprecated	B-General_Concept
due	O
to	O
security	O
concerns	O
and	O
improvements	O
in	O
Web	B-General_Concept
APIs	I-General_Concept
.	O
</s>
<s>
The	O
last	O
one	O
that	O
was	O
officially	O
supported	O
was	O
Adobe	B-Operating_System
Flash	I-Operating_System
Player	I-Operating_System
,	O
which	O
Adobe	O
discontinued	O
in	O
2020	O
.	O
</s>
<s>
Mozilla	B-Operating_System
had	O
no	O
mechanism	O
to	O
restrict	O
the	O
privileges	O
of	O
legacy	O
Firefox	B-Application
extensions	B-Protocol
.	O
</s>
<s>
This	O
meant	O
that	O
a	O
legacy	O
extension	O
could	O
read	O
or	O
modify	O
the	O
data	O
used	O
by	O
another	O
extension	O
or	O
any	O
file	O
accessible	O
to	O
the	O
user	O
running	O
Mozilla	B-Operating_System
applications	B-Application
.	O
</s>
<s>
But	O
the	O
current	O
WebExtensions	B-Operating_System
API	B-General_Concept
imposes	O
many	O
restrictions	O
.	O
</s>
<s>
Starting	O
with	O
Firefox	B-Application
40	O
,	O
Mozilla	B-Operating_System
began	O
to	O
roll	O
out	O
a	O
requirement	O
for	O
extension	O
signing	O
.	O
</s>
<s>
It	O
is	O
now	O
required	O
in	O
all	O
official	O
Firefox	B-Application
releases	O
.	O
</s>
<s>
The	O
Mozilla	B-Operating_System
add-ons	B-Operating_System
website	O
is	O
the	O
official	O
repository	O
for	O
Firefox	B-Operating_System
add-ons	I-Operating_System
.	O
</s>
<s>
In	O
contrast	O
to	O
mozdev.org	B-Protocol
which	O
provides	O
free	O
hosting	O
for	O
Mozilla-related	O
projects	O
,	O
the	O
add-ons	B-Operating_System
site	O
is	O
tailored	O
for	O
users	O
.	O
</s>
<s>
By	O
default	O
,	O
Firefox	B-Application
automatically	O
checks	O
the	O
site	O
for	O
updates	O
to	O
installed	O
add-ons	B-Operating_System
.	O
</s>
<s>
In	O
January	O
2008	O
,	O
Mozilla	B-Operating_System
announced	O
that	O
the	O
site	O
had	O
accumulated	O
a	O
total	O
of	O
600	O
million	O
add-on	B-Operating_System
downloads	O
and	O
that	O
over	O
100	O
million	O
installed	O
add-ons	B-Operating_System
automatically	O
check	O
the	O
site	O
for	O
updates	O
every	O
day	O
.	O
</s>
