Hello again from Dark Cloudy Rainy Indonesia
I have had a good look around your static site which is still very good (even though a bit outdated) and I think can be reasonably re-made to be responsive directly within FF keeping somewhat to the original design), with a little bit of work and thinking about.
Although this could be an opportunity to completely re-design and think about the layout and update some of the content of your web-site I also understand that you would probably be wanting to keep the overall style/design of the existing site as much as possible in order to not over confuse your
elderly clients, although perhaps a little bit more credit should be given as to the adaptability and resourcefulness of all us elderlies

) If the design is clean/modern and the navigation clear and simple then it will be fairly straightforward for
young or old to follow
HOWEVER,
Whichever way you decide, your
existing web-site design is going to need
some re-designing and thinking about, to make it fully functional and visually pleasing in
Responsive mode.
One of the main design changes needed(layout function ability (take your pick) is the use of "Tabs" in some of your pages especially in your "Testimonials" where at the moment there are some with quite a few tabs.
The problem with "Tabs" in responsiveness is they don't respond very well and can look squished untidy or too close together (the almighty Google also don't like links (tabs in this case) to be too close together at smaller device sizes, especially for those of us who have large fingers and unsteady hands (due mostly to old age) as in those devices one doesn't have the luxury of a mouse, so the dangers are hitting the wrong link/tab/button etc, or even worse hitting 2 at the same time.
The new FF 6.(something) is supposed to have a new feature which changes "Tabs" into" accordion" at lower viewports, I haven't yet had the time or opportunity to test it out, but even so I am not sure that would be a clean and proper solution for the best experience for your clients. so this is definitely one part of your design for a responsive site that needs thinking about.
I have done some testing within FF for the use of a modal window idea and so far not had any problems getting all of the links/ images on your pages for all of them to bring up a modal window, (including some of the one's which originally are separate pages (the "About Us" for example I have tried that as a modal ) The pdf files can be placed into the project folder within FF and linked to from there, works a treat, so that shouldn't give you a problem.
I am not sure about the link to the shopping cart for orders, can't seem to get that to work properly, so that might need more thinking about as to how to get that going
The "Help" buttons alongside your page title, don't do anything except go to pages that have no help content at all, these should be removed in your new design (or create some proper help pages).
Some of your content needs updating/freshening up (especially quoting dates etc) a little bit, always has and always will be something that is essential with any web-site is to keep updating content (that includes images etc) in order to keep the almighty Google and the other lemming followers happily indexing your pages
Other than those few things, you shouldn't really have too many problems getting a new responsive site up and running in no time at all looking good and as close to your static design as doesn't matter, by working directly from within FF,
It is still going to take some work, but not nearly as much as trying to alter and change your existing site, and far less danger of mistakes/conflicts going on, and putting static pages into a responsive modal window or any other way is possible, it doesn't make the static page responsive, so trying it that way is about as much use as a chocolate fire guard..
Anyway I hope that sort of helps, I can put together some video's for you to show the process for adding modal window etc if you would find that helpful.
Wayan
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