I'd like to disable some features of a web app I'm building, if the browser is Tor Browser. Can I inside the browser itself (client side, not server side) find out if the browser is Tor Browser?

I would prefer a solution that didn't issue any HTTP requests to match the browser's IP against Tor exit nodes.

Background: In my case, Tor Browser pops up a dialog that asks the user *"Should Tor Browser allow this website to extract HTML5 canvas image data?"*, because, says Tor Browser, canvas image data can be used to uniquely identify a browser.

**Update:** After reading the answers below: Perhaps the best solution in my case, is to keep a list of Tor exit nodes server side (an up-to-date list, refreshed periodically), and when a browser loads the page, I set a variable in a `<script>` tag, if the browser's IP matches such an exit node: `var isProbablyTorBrowser = true`. Then, client side, no additional requests, or complicated logic, is needed.
['https://Stackoverflow.com/questions/28200567', 'https://Stackoverflow.com', 'https://Stackoverflow.com/users/694469/']
The "official" way to detect tor is to check the user's IP address and see if it's a tor exit node. Tor runs [TorDNSEL](https://www.torproject.org/projects/tordnsel.html.en) for this purpose.

Here's a PHP implementation of a TorDNSEL lookup from a tutorial by [Irongeek](http://www.irongeek.com/i.php?page=security/detect-tor-exit-node-in-php)

```
function IsTorExitPoint(){
    if (gethostbyname(ReverseIPOctets($_SERVER['REMOTE_ADDR']).".".$_SERVER['SERVER_PORT'].".".ReverseIPOctets($_SERVER['SERVER_ADDR']).".ip-port.exitlist.torproject.org")=="127.0.0.2") {
        return true;
    } else {
        return false;
    } 
}

function ReverseIPOctets($inputip){
    $ipoc = explode(".",$inputip);
    return $ipoc[3].".".$ipoc[2].".".$ipoc[1].".".$ipoc[0];
}

```

If you're not using PHP, you should still be able to adapt this relatively easily.

***Another method of detecting Tor is to have a script download the list of Tor exit nodes every half hour or so, then check each user's IP address against that list. This may be less reliable, though, as not all exit nodes are published. There's a list you can use, and instructions, available at [dan.me.uk](https://www.dan.me.uk/tornodes).***

EDIT: Since you updated your question, the second option (a list you host locally) is going to be preferable.
By the firefox resource bundle you can check it.

The resource:// URI scheme is used by Firefox to call on-disk resources from internal modules and extensions.

But some of these resources may also be included to any web page and executed via script tag. Mozilla developers is not consider the resources as a fingerprinting vector, despite the fact that some of them can reveal what the user does not wish. For example, differences in built-in preferences files clearly indicates you are using Windows or Linux or Mac, even if you're behind Tor Browser.

<https://www.browserleaks.com/firefox>