I visited this WHS in July 2019 on several occasions before and after the 2019 WH meetup in Scotland. I visited from both sides of the Firth of Forth as well as from different vantage points such as the Forth Bridges Viewpoint (nothing special) and the view from the Forth Road Bridge.
For the latter, you have to sign in as a pedestrian or a cyclist and you can cross the road bridge with great views of the inscribed Forth Bridge. The true reason I visited the Forth Road Bridge was that I had read on the internet that a new UNESCO world heritage inscription plaque had been unveiled here. However, to my dismay, at the visitor centre I was informed that apparently UNESCO was against the plaque being put there and it was removed. Now there is another small UNESCO WHS plaque just behind the North Queensferry South Bay bus stop next to the inscribed bridge.
Designed by Sir John Fowler and Sir Benjamin Baker, construction of the Forth Bridge began in 1882 and it was officially opened in 1890. Creating the bridge consumed 55,000 tonnes of steel, 6.5 million rivets and 640,000 cubic feet of granite. There was also a human cost with at least 73 people known to have died while constructing the bridge and its approaches. When completed, Forth Bridge was the longest single cantilever railway bridge in the world. In 1963 the Forth Road Bridge was opened and it was the longest span suspension bridge in Europe at the time.
My favourite viewpoints were definitely at sunset from South Queensferry, either from the pier (photo) with many sea gulls ready to eat your fish and chips, or just on the rocks in front of the closest house to the water (to get there take the first alley on the left after the pier and squeeze yourself through the overgrown grass next to the rubble wall). Forth Bridge was also visible from Calton Hill in Edinburgh even though I wouldn't count it as a visit from there. If you're keen to climb to the top of the iconic bridge, apparently occasionally tours with hard hats are being organised via a small lift which takes you above the railway. However, plans are currently underway to expand this possibility to create a similar experience to Sydney's Harbour Bridge Climb. If you're not that adventurous, you can either catch a train to/from Glasgow/Edinburgh or take a boat trip too.
As a souvenir, I managed to find a Polymer Series £5 Clydesdale Bank note issued by the Bank of Scotland. All in all, I really enjoyed my visit which was on par if not a bit better than my visit to Vizcaya Bridge in Spain.