I have long experience in both branding and photo retouching (+15 years) as well as websites design (+10 years). I've opened my own studio and used the Adobe suit extensively. Five years ago, I dropped all the Adobe stuff to rely exclusively on open-source software on a Linux box. Works like a charm, as long as I don't need co-workers to intervene on the files (getting people to switch from their beloved Photoshop is hard). Even then, it's possible, just a bit cumbersome.
I use:
To answer your question more directly:
To design a UI, I'd use Inkscape for a quick sketch, then implement it directly in the browser, where a responsive design can take place. If I don't want to code anything, the web provides a number of application (both free and subscription based) to design a web UI, a Google search for "design UI mockup online" will provide results.
To design UI components, I'd use either Inkscape, if I need to export at different resolutions, or Gimp/Krita if I don't.