Added way to set Firefox preferences via environment variables.

README.md

@@ -39,6 +39,7 @@ Mozilla Firefox is a free and open-source web browser developed by Mozilla Found
       * [Shell Access](#shell-access)
       * [Increasing Shared Memory Size](#increasing-shared-memory-size)
       * [Sound Support](#sound-support)
+      * [Setting Firefox Preferences Via Environment Variables](#setting-firefox-preferences-via-environment-variables)
       * [Troubleshooting](#troubleshooting)
          * [Crashes](#crashes)
       * [Support or Contact](#support-or-contact)
@@ -460,6 +461,42 @@ For Firefox to be able to use the audio device available on
 the host, `/dev/snd` must be exposed to the container by adding the
 `--device /dev/snd` parameter to the `docker run` command.
 
+## Setting Firefox Preferences Via Environment Variables
+
+Firefox preferences can be set via environment variables
+passed to the containter.  During the startup, a script process all these
+variables and modify the preference file accordingly.
+
+The name of the environment variable must start with `FF_PREF_`, followed by a
+string of your choice.  For example, `FF_PREF_MY_PREF` is a valid name.
+
+The content of the variable should be in the format `NAME=VAL`, where `NAME` is
+the name of the preference (as found in the `about:config` page) and `VAL` is
+its value.  A value can be one of the following type:
+  - string
+  - integer
+  - boolean
+It is important to note that a value of type `string` should be surrounded by
+double quotes.  Other types don't need them.
+
+For example, to set the `network.proxy.http` preference, one would pass the
+environment variable to the container by adding the following argument to the
+`docker run` command:
+
+```
+-e "FF_PREF_HTTP_PROXY=network.proxy.http=\"proxy.example.com\""
+```
+
+If a preference needs to be *removed*, its value should be set to `UNSET`.  For
+example:
+
+```
+-e "FF_PREF_HTTP_PROXY=network.proxy.http=UNSET
+```
+
+**NOTE**: This is an advanced usage and it is recommended to set preferences
+via Firefox directly.
+
 ## Troubleshooting
 
 ### Crashes

appdefs.xml

@@ -49,6 +49,44 @@ the host, `/dev/snd` must be exposed to the container by adding the
 `--device /dev/snd` parameter to the `docker run` command.
       </content>
     </section>
+      <section>
+        <title level="2">Setting {{ defs.app.friendly_name }} Preferences Via Environment Variables</title>
+        <content>
+{{ defs.app.friendly_name }} preferences can be set via environment variables
+passed to the containter.  During the startup, a script process all these
+variables and modify the preference file accordingly.
+
+The name of the environment variable must start with `FF_PREF_`, followed by a
+string of your choice.  For example, `FF_PREF_MY_PREF` is a valid name.
+
+The content of the variable should be in the format `NAME=VAL`, where `NAME` is
+the name of the preference (as found in the `about:config` page) and `VAL` is
+its value.  A value can be one of the following type:
+  - string
+  - integer
+  - boolean
+It is important to note that a value of type `string` should be surrounded by
+double quotes.  Other types don't need them.
+
+For example, to set the `network.proxy.http` preference, one would pass the
+environment variable to the container by adding the following argument to the
+`docker run` command:
+
+```
+-e "FF_PREF_HTTP_PROXY=network.proxy.http=\"proxy.example.com\""
+```
+
+If a preference needs to be *removed*, its value should be set to `UNSET`.  For
+example:
+
+```
+-e "FF_PREF_HTTP_PROXY=network.proxy.http=UNSET
+```
+
+**NOTE**: This is an advanced usage and it is recommended to set preferences
+via {{ defs.app.friendly_name }} directly.
+        </content>
+      </section>
       <section>
         <title level="2">Troubleshooting</title>
         <content/>

rootfs/etc/cont-init.d/firefox-set-prefs-from-env.sh

@@ -0,0 +1,48 @@
+#!/usr/bin/with-contenv sh
+
+set -e
+
+log() {
+    echo "[cont-init.d] $(basename $0): $*"
+}
+
+PREF_FILE="${1:-/config/profile/prefs.js}"
+
+if [ -z "$PREF_FILE" ]; then
+    log "ERROR: Preference file not set."
+    exit 1
+fi
+
+mkdir -p "$(dirname "$PREF_FILE")"
+[ -f "$PREF_FILE" ] || touch "$PREF_FILE"
+
+env | grep "^FF_PREF_" | while read ENV
+do
+    ENAME="$(echo "$ENV" | cut -d '=' -f1)"
+    EVAL="$(echo "$ENV" | cut -d '=' -f2-)"
+
+    if [ -z "$EVAL" ]; then
+        log "Skipping environment variable '$ENAME': no value set."
+        continue
+    fi
+
+    if echo "$EVAL" | grep -q "="; then
+        PNAME="$(echo "$EVAL" | cut -d '=' -f1)"
+        PVAL="$(echo "$EVAL" | cut -d '=' -f2-)"
+        [ -n "$PVAL" ] || PVAL='""'
+    else
+        PNAME="$EVAL"
+        PVAL='""'
+    fi 
+
+    if [ "$PVAL" = "UNSET" ]; then
+        log "Removing preference '$PNAME'..."
+        sed -i "/user_pref(\"$PNAME\",.*);/d" "$PREF_FILE"
+    elif grep -q "user_pref(\"$PNAME\"," "$PREF_FILE"; then
+        log "Setting preference '$PNAME'..."
+        sed -i "s/user_pref(\"$PNAME\",.*);/user_pref(\"$PNAME\", $PVAL);/" "$PREF_FILE"
+    else
+        log "Setting new preference '$PNAME'..."
+        echo "user_pref(\"$PNAME\", $PVAL);" >> "$PREF_FILE"
+    fi
+done