<s>
A	O
browser	B-Protocol
extension	I-Protocol
is	O
a	O
small	O
software	O
module	O
for	O
customizing	O
a	O
web	B-Application
browser	I-Application
.	O
</s>
<s>
Browsers	B-Application
typically	O
allow	O
a	O
variety	O
of	O
extensions	O
,	O
including	O
user	B-Application
interface	I-Application
modifications	O
,	O
cookie	B-Application
management	O
,	O
ad	O
blocking	O
,	O
and	O
the	O
custom	O
scripting	B-Language
and	O
styling	O
of	O
web	O
pages	O
.	O
</s>
<s>
Browser	B-Protocol
plug-ins	I-Protocol
are	O
a	O
separate	O
type	O
of	O
module	O
.	O
</s>
<s>
The	O
main	O
difference	O
is	O
that	O
extensions	O
are	O
distributed	O
as	O
source	O
code	O
,	O
while	O
plug-ins	B-Application
are	O
distributed	O
as	O
executables	B-Application
(	O
i.e.	O
</s>
<s>
object	B-Language
code	I-Language
)	O
.	O
</s>
<s>
Plug-ins	B-Application
are	O
no	O
longer	O
supported	O
by	O
the	O
major	O
browsers	B-Application
,	O
but	O
extensions	O
are	O
widely	O
used	O
.	O
</s>
<s>
The	O
most	O
popular	O
browser	B-Application
,	O
Google	B-Application
Chrome	I-Application
,	O
has	O
over	O
100,000	O
extensions	O
available	O
but	O
stopped	O
supporting	O
plug-ins	B-Application
in	O
2020	O
.	O
</s>
<s>
Internet	B-Application
Explorer	I-Application
was	O
the	O
first	O
major	O
browser	B-Application
to	O
support	O
extensions	O
,	O
with	O
the	O
release	O
of	O
version	B-Protocol
4	I-Protocol
in	O
1999	O
.	O
</s>
<s>
Firefox	B-Application
has	O
supported	O
extensions	O
since	O
its	O
launch	O
in	O
2004	O
.	O
</s>
<s>
Opera	B-Application
began	O
supporting	O
extensions	O
in	O
2009	O
,	O
and	O
both	O
Google	B-Application
Chrome	I-Application
and	O
Safari	B-Application
did	O
so	O
the	O
following	O
year	O
.	O
</s>
<s>
Microsoft	B-Application
Edge	I-Application
added	O
extension	O
support	O
in	O
2016	O
.	O
</s>
<s>
In	O
2015	O
,	O
a	O
community	O
working	O
group	O
formed	O
under	O
the	O
W3C	O
to	O
create	O
a	O
single	O
standard	O
application	B-Application
programming	I-Application
interface	I-Application
(	O
API	B-Application
)	O
for	O
browser	B-Protocol
extensions	I-Protocol
.	O
</s>
<s>
While	O
that	O
goal	O
is	O
unlikely	O
to	O
be	O
achieved	O
,	O
the	O
majority	O
of	O
browsers	B-Application
already	O
use	O
the	O
same	O
or	O
very	O
similar	O
APIs	B-Application
due	O
to	O
the	O
popularity	O
of	O
Google	B-Application
Chrome	I-Application
.	O
</s>
<s>
Chrome	B-Application
was	O
the	O
first	O
browser	B-Application
with	O
an	O
extension	O
API	B-Application
based	O
solely	O
on	O
HTML	B-Language
,	O
CSS	B-Language
,	O
and	O
JavaScript	B-Language
.	O
</s>
<s>
Beta	O
testing	O
for	O
this	O
capability	O
began	O
in	O
2009	O
,	O
and	O
the	O
following	O
year	O
Google	B-Application
opened	O
the	O
Chrome	B-Application
Web	I-Application
Store	I-Application
.	O
</s>
<s>
In	O
the	O
same	O
year	O
,	O
Chrome	B-Application
overtook	O
Internet	B-Application
Explorer	I-Application
as	O
the	O
world	O
's	O
most	O
popular	O
browser	B-Application
,	O
and	O
its	O
market	B-Application
share	I-Application
continued	O
to	O
grow	O
,	O
reaching	O
60%	O
in	O
2018	O
.	O
</s>
<s>
Because	O
of	O
Chrome	B-Application
's	O
success	O
,	O
Microsoft	O
created	O
a	O
very	O
similar	O
extension	O
API	B-Application
for	O
its	O
Edge	B-Application
browser	I-Application
,	O
with	O
the	O
goal	O
of	O
making	O
it	O
easy	O
for	O
Chrome	B-Application
extension	O
developers	O
to	O
port	O
their	O
work	O
to	O
Edge	B-Application
.	O
</s>
<s>
But	O
after	O
three	O
years	O
Edge	B-Application
still	O
had	O
a	O
disappointingly	O
small	O
market	B-Application
share	I-Application
,	O
so	O
Microsoft	O
rebuilt	O
it	O
as	O
a	O
Chromium-based	O
browser	B-Application
.	O
</s>
<s>
(	O
Chromium	B-Language
is	O
Google	B-Application
's	I-Application
open-source	O
project	O
that	O
serves	O
as	O
the	O
functional	O
core	O
of	O
Chrome	B-Application
and	O
many	O
other	O
browsers	B-Application
.	O
)	O
</s>
<s>
Now	O
that	O
Edge	B-Application
has	O
the	O
same	O
API	B-Application
as	O
Chrome	B-Application
,	O
extensions	O
can	O
be	O
installed	O
directly	O
from	O
the	O
Chrome	B-Application
Web	I-Application
Store	I-Application
.	O
</s>
<s>
With	O
its	O
own	O
market	B-Application
share	I-Application
in	O
decline	O
,	O
Mozilla	B-Operating_System
also	O
decided	O
to	O
conform	O
.	O
</s>
<s>
In	O
2015	O
,	O
the	O
organization	O
announced	O
that	O
the	O
long-standing	O
XUL	B-Application
and	O
XPCOM	B-Application
extension	O
capabilities	O
of	O
Firefox	B-Application
would	O
be	O
replaced	O
with	O
a	O
less-permissive	O
API	B-Application
very	O
similar	O
to	O
Chrome	B-Application
's	O
.	O
</s>
<s>
Firefox	B-Application
extensions	O
are	O
now	O
largely	O
compatible	O
with	O
their	O
Chrome	B-Application
counterparts	O
.	O
</s>
<s>
Until	O
2020	O
,	O
Apple	O
was	O
the	O
lone	O
major	O
exception	O
to	O
this	O
trend	O
,	O
but	O
with	O
the	O
release	O
of	O
Safari	B-Application
14	O
for	O
macOS	O
,	O
the	O
browser	B-Application
added	O
support	O
for	O
extensions	O
conforming	O
to	O
the	O
Chrome	B-Application
API	B-Application
.	O
</s>
<s>
Browser	B-Protocol
extensions	I-Protocol
typically	O
have	O
access	O
to	O
sensitive	O
data	O
,	O
such	O
as	O
browsing	B-Protocol
history	I-Protocol
,	O
and	O
they	O
have	O
the	O
ability	O
to	O
alter	O
some	O
browser	B-Application
settings	O
,	O
add	O
user	B-Application
interface	I-Application
items	O
,	O
or	O
replace	O
website	O
content	O
.	O
</s>
<s>
There	O
have	O
also	O
been	O
cases	O
of	O
applications	B-Application
installing	O
browser	B-Protocol
extensions	I-Protocol
without	B-Application
the	I-Application
user	I-Application
's	I-Application
knowledge	I-Application
,	O
making	O
it	O
hard	O
for	O
the	O
user	O
to	O
uninstall	O
the	O
unwanted	O
extension	O
.	O
</s>
<s>
Some	O
Google	B-Application
Chrome	I-Application
extension	O
developers	O
have	O
sold	O
their	O
extensions	O
to	O
third-parties	O
who	O
then	O
incorporated	O
adware	O
.	O
</s>
<s>
In	O
2014	O
,	O
Google	B-Application
removed	O
two	O
such	O
extensions	O
from	O
the	O
Chrome	B-Application
Web	I-Application
Store	I-Application
after	O
many	O
users	O
complained	O
about	O
unwanted	O
pop-up	O
ads	O
.	O
</s>
<s>
The	O
following	O
year	O
,	O
Google	B-Application
acknowledged	O
that	O
about	O
five	O
percent	O
of	O
visits	O
to	O
its	O
own	O
websites	O
had	O
been	O
altered	O
by	O
extensions	O
with	O
adware	O
.	O
</s>
