L10n:Tools
Several tools can be used to localize Mozilla-based products. One approach is to use Mozilla Translator. The other one is to use any UTF-8-aware text editor. In any case, you'll have to connect to the CVS repository before and after this task. You can also use an IDE with CVS support, like the Eclipse platform (see below).
Using the Eclipse platform
The Eclipse platform (about 100MB download, Java Runtime Environment required) has an integrated CVS support. If you have a l10n CVS account you can use it to work directly on your l10n source files. Eclipse doesn't support natively the kind of SSH keys used by cvs.mozilla.org though. You'll need a running external SSH agent while you're working on it.
Linux
Use the script provided on the CVS-SSH FAQ (assuming your private key is ~/.ssh/id_dsa):
eval `ssh-agent -s` ssh-add ~/.ssh/id_dsa $SHELL ssh-agent -k exit
Windows
You can use the pageant tool provided on putty's website. Have it loaded in the taskbar, click on "Add Key" and pick your private key file.
Additionaly, you'll have to configure eclipse to use plink[1]. You can either use the one provided with TortoiseCVS (which should be located at C:\Program Files\TortoiseCVS\TortoisePlink.exe) or the one coming with putty (see above).
To do so, open eclipse and go to "Window > Preferences > Team > CVS > Ext connection method". For CVS_RSH, hit the browse button and find (Tortoise)Plink.exe. The other settings are fine at their defaults. Create a new Repository Location using ext, not extssh. Connect to cvs.mozilla.org and checkout your project.
Editing .properties files
While eclipse can natively open .properties files, they will appear in their ASCII form which is not very pretty if your language uses other Unicode characters. You can install the Properties editor plugin which provides additional syntax highlighting and handles the Unicode conversion. Follow these instructions to install the plugin.