My recommendation won't fit all your must-have requirements, but as far as I know its the best thats out there that comes close so I want to show it anyway.
I've used Balsamic Mockups in the past to mock up web applications, so I think it will be good to mock up websites as well. But keep that in mind, I'm not a web designer but a web app developer.
Unfortunately it is not a full-color-mockup tool but a wireframe thing so it won't do if you really need this.
You can try the webapp in action on User Experience Stack Exchange: in a question or answer edit box, use the “UI wireframe” tool (Ctrl+M).
- Windows or Linux YES, both.
- Desktop Application/Browser Based YES, both depending on your requirements.
- Full-color mockups as opposed to wireframes NO, This is a wireframe tool.
- Price not an issue if it does the job ... This is a commercial tool with various pricing depending on the use-case. Not expensive from my point of view.
- Still supported YES. Latest version is from March 2014, the company is up and alive with good prospects to be around for a while.
- Something a step up from Inkscape YES. At least from my point of view.
- Interactivity/linked pages support would be nice YES You can link mockups.
- Any other kind of mockup generation like general UI interfaces or documents. YES This is the reason I post this recommendation even though it doesn't match all your required entries.
I took that image from their homepage as I don't have an installation myself anymore
When I was developing web applications a few years back (~2012/2011 or so) I used Balsamic intensively in the pre-code phase to get the users and developers on the same page. It worked great in producing a common idea of what was needed and how it would look later on. We didn't continue using the mockups once the tool was in its early betas but we had the UI ready (directly coded from the Basamic by a human into Flex) before the program so the devs "just" needed to add the flesh part by part.
That had the upshot of users that felt that everything was on the way to how they wanted it the whole beta phase and the improvement iterations were quite short so everything felt moving all the time.
It had the downside that the users always thought the program would be ready next week because the "hard part" (defining how it should look like) was already done. It took some time to explain to them why "just adding that button" took three weeks.