Firefox only (Win/Mac/Linux): Firefox extension FlagTab adds relative tabs and customisable colours so you can keep your tabs organised more easily.
Once you've installed the extension, you can colourise a tab by simply Ctrl+Clicking to cycle through the colours. Once you've colourised a tab, the child tabs will have the same colour as the parent tab — it's similar to the tab grouping feature in Internet Explorer 8. You can access the colour settings through the tab context menu to directly set a colour, or clear the tab colourisation. If the tab colours are too drastic for you, there's also an option to only colourise the tab icon, instead of the background. FlagTab is a free download, works anywhere Firefox does.