# Targeting Different Browsers
When building an extension with WXT, you can create multiple builds of your extension targeting different browsers and manifest versions.
## Target a Browser
Use the `-b` CLI flag to create a separate build of your extension for a specific browser. By default, `chrome` is targeted.
```sh
wxt # same as: wxt -b chrome
wxt -b firefox
wxt -b custom
```
During development, if you target Firefox, Firefox will open. All other strings open Chrome by default. To customize which browsers open, see [Set Browser Binaries](/guide/essentials/config/browser-startup#set-browser-binaries).
Additionally, WXT defines several constants you can use at runtime to detect which browser is in use:
```ts
if (import.meta.env.BROWSER === 'firefox') {
console.log('Do something only in Firefox builds');
}
if (import.meta.env.FIREFOX) {
// Shorthand, equivalent to the if-statement above
}
```
Read about [Built-in Environment Variables](/guide/essentials/config/environment-variables.html#built-in-environment-variables) for more details.
## Target a Manifest Version
To target specific manifest versions, use the `--mv2` or `--mv3` CLI flags.
:::tip Default Manifest Version
By default, WXT will target MV2 for Safari and Firefox and MV3 for all other browsers.
:::
Similar to the browser, you can get the target manifest version at runtime using the [built-in environment variable](/guide/essentials/config/environment-variables.html#built-in-environment-variables):
```ts
if (import.meta.env.MANIFEST_VERSION === 2) {
console.log('Do something only in MV2 builds');
}
```
## Filtering Entrypoints
Every entrypoint can be included or excluded when targeting specific browsers via the `include` and `exclude` options.
Here are some examples:
- Content script only built when targeting `firefox`:
```ts
export default defineContentScript({
include: ['firefox'],
main(ctx) {
// ...
},
});
```
- HTML file only built for all targets other than `chrome`:
```html
```
Alternatively, you can use the [`filterEntrypoints` config](/api/reference/wxt/interfaces/InlineConfig#filterentrypoints) to list all the entrypoints you want to build.