diff --git a/python/mach/bash-completion.sh b/python/mach/bash-completion.sh
index 13935cf88cd2..82dc9fb0f515 100644
--- a/python/mach/bash-completion.sh
+++ b/python/mach/bash-completion.sh
@@ -1,3 +1,8 @@
+# Is bash completion too slow to your taste?
+# Consider generating fast completion scripts with:
+# ./mach mach-completion bash -f path/to/mach.bash-completion
+# See https://firefox-source-docs.mozilla.org/mach/usage.html#tab-completion
+
 function _mach()
 {
   local cur targets
diff --git a/python/mach/mach/commands/commandinfo.py b/python/mach/mach/commands/commandinfo.py
index 414de9fd10b2..f31c36a8472b 100644
--- a/python/mach/mach/commands/commandinfo.py
+++ b/python/mach/mach/commands/commandinfo.py
@@ -230,7 +230,12 @@ def _zsh_describe(value, description=None):
     help="File path to save completion script.",
 )
 def completion_bash(command_context, outfile):
-    commands_subcommands = []
+    # Note: Make sure to generate a Bash 3-compatible script, because macOS
+    # ships with Bash 3 by default. One of the consequences is that Bash 4
+    # features such as associative arrays ("declare -A") cannot be used.
+    # For example, while completion_zsh uses the associative array syntax for
+    # commands_subcommands, here we use case/esac instead.
+    case_commands_subcommands = []
     case_options = []
     case_subcommands = []
     for i, cmd in enumerate(commands_info(command_context)):
@@ -274,9 +279,15 @@ def completion_bash(command_context, outfile):
         # Build case statement for subcommands.
         subcommands = [_zsh_describe(s.subcommand, None) for s in cmd.subcommands]
         if subcommands:
-            commands_subcommands.append(
-                '[{}]=" {} "'.format(
-                    cmd.name, " ".join([h.subcommand for h in cmd.subcommands])
+            comsubs = " ".join([h.subcommand for h in cmd.subcommands])
+            case_commands_subcommands.append(
+                "\n".join(
+                    [
+                        "            ({})".format(cmd.name),
+                        '            comsubs=" {} "'.format(comsubs),
+                        "            ;;",
+                        "",
+                    ]
                 )
             )
 
@@ -295,10 +306,10 @@ def completion_bash(command_context, outfile):
         opt for opt_strs in global_options(command_context) for opt in opt_strs
     ]
     context = {
+        "case_commands_subcommands": "\n".join(sorted(case_commands_subcommands)),
         "case_options": "\n".join(case_options),
         "case_subcommands": "\n".join(case_subcommands),
         "commands": " ".join(commands(command_context)),
-        "commands_subcommands": " ".join(sorted(commands_subcommands)),
         "globalopts": " ".join(sorted(globalopts)),
     }
 
diff --git a/python/mach/mach/commands/completion_templates/bash.template b/python/mach/mach/commands/completion_templates/bash.template
index 53723087024e..6c9c002d6caa 100644
--- a/python/mach/mach/commands/completion_templates/bash.template
+++ b/python/mach/mach/commands/completion_templates/bash.template
@@ -2,7 +2,6 @@ _mach_complete()
 {
     local com coms comsubs cur opts script sub subs
     COMPREPLY=()
-    declare -A comsubs=( %(commands_subcommands)s )
 
     _get_comp_words_by_ref -n : cur words
     # for an alias, get the real script behind it
@@ -19,7 +18,10 @@ _mach_complete()
 
         if [[ -z $com ]]; then
             com=$word
-        elif [[ "${comsubs[$com]}" == *" $word "* ]]; then
+            case $com in
+%(case_commands_subcommands)s
+            esac
+        elif [[ $comsubs == *" $word "* ]]; then
             sub=$word
             break
         fi