If you want to use the extension on SeaMonkey you will need xSidebar extension(version 0.4 is currently not working in SeaMonkey - will fix as soon as possible).
After the installation (and Firefox restart!) Customize your toolbar and add the new button "Files". If you are using SeaMonkey you can't do that but don't worry clicking anywhere in the tree will open the file view(versions below 0.4).
Before you can use the tree in the sidebar you must go to Tools->Add-ons->Firefly preferences and add what you want to see in the sidebar(in v0.4 Firefly tries to guess some of your devices, but you may still have to enter some or all of them).
Some users reported that nothing appears in the sidebar after the tree is set up for the first time and a restart is required to work - this is most likely caused by another extension like All-In-One sidebar - because this extension just hides the sidebar and not closing it Firefly can't get the new data. In v0.4 there is a reload button that will get the changes.
Since it seems that 'moz-icon' is not working on Linux (and that's what i use to show file icons) you can customize what icons you want to see for what type of files in the preferences (please use 16x16 and 32x32 images). In that case you must turn on the preference to show internal icons in the Globals section of the preferences.
If you want to be able to preview Audio/Video,document and pdf files you will need VideoLan Client,OpenOffice and Adobe Acrobat plugins respectively and they must work correctly. In v0.4 there is support also for Mplayer, Windows Media Player and QuickTime
You can also view sites with IE's engine but for that purpose you will need to have IETab extension installed and working correctly(it will work only on Windows of course). In v0.4 HTML views (Mozilla and IE) are temporary not available. They will be added back as soon as possible.