Some random thoughts:
1) Other sources of mercury would include a broken thermometer, or mercury for an artificial horizon, or gun cartridge primers, or gun cleaning materials. The procedure for cleaning the British .303 Enfield requires pouring boiling hot water down the barrel, washing out the dissolved residue, for example. That was followed by swabbing with oiling by pulling a swab through. I would expect the resulting mercury compound to be distinctly different from that found in freckle cream.
2) Scratches on the interior bottom might be from a knife used to scrape out contents (think of your experience with peanut butter). That implies that the material was important, and that it didn't "pour". Another idea, built on item 1) is use of a jar to hold the oily cleaning lanyard and metal jag used for oiling a gun after cleaning. The metal jag or swab might scratch the glass.
3) White glass looks “medicinal” and pure, giving an impression of sanitary cleanliness. Clear glass shows off the contents, to appeal to potential buyers. What sort of thing would be easier to sell by appearance in a glass jar? Food items obviously (olives, cocktail onions, baby food), but non-food items as well. What is the color or appearance of freckle cream? Clear glass also shows the quantity of the contents. Is that a clue?
4) A high refraction index does not by itself protect from UV. However, high index glass is eye-catching, which is another marketing aspect that gives a clue to the type of contents.