Tools
Semagic
Semagic is a LiveJournal (and other journaling sites) posting clients. You can use it to post to your journals or communities you are a member of, write post in advance and save them for later, set it to autosave your posts in progress, manage your friends and the order of your posts, and more! It also has several easy to use formatting options to make your text bold, italic, bigger, smaller, centered, a different colour and font, etc. There are also easy to install spellcheckers in different languages. I use it on Windows, but according to the site, it can be used with Mac and Linux.
Firefox Extensions
Adblock Plus
No paid account? No problem! Adblock Plus will make all those annoying ads disappear right before your eyes, and not just on Livejournal either!
BBCode
Don't know the HTML code for coloured text, small text, links, images, etc? BBCode can do all that for you. Just activate the HTML code in the options. As a bonus, here are custom tags you can enter to automatically do the LJ username or the LJ community link. Just write the name of the journal, highlight and select the custom tag you want:
- LJ user: <lj user="_value_">
- LJ community: <lj comm="_value_">
Lazarus
Lazarus allows you to recover that long emotional tag you just made that the LJ server ate.
Livejournal Addons
Two words: Thread. Unfolding. Even if you don't have a paid account! It does more things too, but, for RP, that is the one that seems most important.
Livejournal Hook
Livejournal Hook works like BBCode, only it only works on Livejournal sites and has more Livejournal-centric functions.
LJLogin
LJLogin is a Firefox extension (so yeah, you'll need to use the Firefox browser) that allows you to change which journal you are logged into on the fly. It sits in your status bar and allows you to pick which journals you want to log in as with a pull-up menu, and if you chose so in the preferences, it'll even automatically reload the page so you can start tagging! An absolutely essential tool for people with several characters.
Stylish
Stylish allows you the change the CSS of websites; in other word, the look and feel of them. How? Well, for example, by making sure that LJ comments never do that annoying one word per line column thing when the thread gets long.
Text-Area Resizer and Mover
Text Area Resizer & Mover allows you to make that small text area bigger so you can see everything you wrote at a glance.