Explain that update_responses needs a channel and removed the URL+certificate patches, in favor of the config option

Updating.md

@@ -125,42 +125,16 @@ Testing the updater (and update process) is non-trivial. There are three separat
 
 In addition, you must [create a new public key pair](https://gitlab.torproject.org/tpo/applications/tor-browser-spec/-/blob/main/processes/KeyGeneration#L41) for signing mar files. Copy the resulting public certificate  `marsigner.der` for later use, too.
 
-## Updater URL
-
-Testing the `nightly` channel is easiest. The URL can be modified in the [build](https://gitlab.torproject.org/tpo/applications/tor-browser-build/-/blob/main/projects/firefox/build#L105) script. If you are testing `release` or `alpha` then a more complicated patch will be needed.
-
-The URL you are setting will contain the update responses, that are some XML files that the Firefox updater polls with a certain frequency, to check if any update is available.
-
-## Signature Public Certificate
-
-After creating the new testing key pair, add the new certificate into tor-browser by overwriting the appropriate secondary key. For `nightly`, you can achieve this with a patch
-```patch
-diff --git a/projects/firefox/build b/projects/firefox/build
-index 732da0f36d52..354145dece4d 100644
---- a/projects/firefox/build
-+++ b/projects/firefox/build
-@@ -102,7 +102,10 @@ tar -C browser/extensions -xf $rootdir/[% c('input_files_by_name/tor-launcher')
- 
- [% IF c("var/nightly") -%]
-   # Set update url for nightly (#33402 / #40033)
--  sed -i 's|^URL=https://aus1\.torproject\.org/.*|URL=https://nightlies.tbb.torproject.org/nightly-updates/updates/nightly-[% c("var/osname") %]/%CHANNEL%/%BUILD_TARGET%/%VERSION%/%LOCALE%|' build/application.ini.in
-+  #sed -i 's|^URL=https://aus1\.torproject\.org/.*|URL=https://nightlies.tbb.torproject.org/nightly-updates/updates/nightly-[% c("var/osname") %]/%CHANNEL%/%BUILD_TARGET%/%VERSION%/%LOCALE%|' build/application.ini.in
-+  sed -i 's|^URL=https://aus1\.torproject\.org/.*|URL=https://people.torproject.org/~sysrqb/update_3/%CHANNEL%/%BUILD_TARGET%/%VERSION%/ALL|' build/application.ini.in
-+
-+  cp $rootdir/marsigner.der toolkit/mozapps/update/updater/nightly_aurora_level3_secondary.der
- [% END -%]
- 
- rm -f configure
-diff --git a/projects/firefox/config b/projects/firefox/config
-index 006c3c9d7c68..fa59b712776e 100644
---- a/projects/firefox/config
-+++ b/projects/firefox/config
-@@ -168,3 +170,4 @@ input_files:
-     # TorButton patch authored by Arthur Edelstein, from https://github.com/arthuredelstein/torbutton/ branch 2.1.10-namecoin
-   - filename: namecoin-etld.patch
-     enable: '[% c("var/namecoin") %]'
-+  - filename: marsigner.der
-```
+## Updater URL and Signature Public Certificate
+
+Testing the `nightly` channel is easiest.
+
+You can change the URL in the [build configuration](https://gitlab.torproject.org/tpo/applications/tor-browser-build/-/blob/main/projects/firefox/config#L38): you just need to set `override_updater_url`.
+This URL will contain the update responses, that are some XML files that the Firefox updater polls with a certain frequency to check if any update is available.
+
+Setting a custom updater URL will also override the update certificate: copy yours to `project/firefox/marsigner.der`.
+
+If you are testing `release` or `alpha` then a more complicated patch will be needed.
 
 ## Sign MAR file
 
@@ -257,9 +231,11 @@ Before creating the update responses, **be sure of having signed the `.mar`s**:
 If you have done that, creating the update responses is very easy, just run this command:
 
 ```sh
-./update_responses
+./update_responses nightly
 ```
 
+If you are on another channel, you should replace nightly with that.
+
 When it is done, copy the content of the `htdocs` directory to the URL you provided to the build (e.g., `~/public_html/update_3`):
 
 ```shell