Starting the REPL
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Go to Firefox Tools menu (to make it visible, press F10 or Alt-T – depening on your platform) and select
Start MozRepl. MozRepl is now listening for connections on the default port 4242. Alternatively, use separate Firefox instance as described below in chapter Starting on specific port.
See platform notes below for how to start Firefox using the REPL addon.
Open the command shell of your operating system and type:
telnet localhost 4242
You’ll be presented with the prompt:
If you’re using Microsoft’s telnet for Windows, it’s probably set to handle one-character-at-a-time instead of one-line-at-a-time. See the Troubleshooting page, or just use netcat (recommended) or putty.
If you’re on Linux or some other Unix variant, you can use
rlwrap to get command history and other niceties. Use arrow-up/down to retrieve previous commands and
Control+R to search among them.
$ rlwrap telnet localhost 4242
If you’re using Emacs, see the Emacs integration page.
Starting on a specific port
To start the repl via the command line on a specific port, execute the following command depending on your platform.
/Applications/Firefox.app/Contents/MacOS/firefox -profile /path/to/profile/folder -repl -repl 7070
/usr/bin/firefox -profile /path/to/profile/folder -repl -repl 7070
"C:\Program Files (x86)\Mozilla Firefox\firefox.exe" /repl /repl 4242
The profile path in the examples above must be a profile that has the mozrepl extension installed.