I find the timeline a bit murky- I could have sworn I was relying on tabbed browsing in Mozilla m12 in 1999- http://beta.slashdot.org/story/8938, it was the single feature that caused me to switch.
The 1995 date in the article definitely preceeds 1999, but the path to getting it into Mozilla does not seem correct.
The linked "directly responsible" more or less meets my memory - MultiZilla first, Mozilla main later. The MultiZilla site of the time confirms it, the history links point to a 2001 start which would make 2002 in Mozilla itself reasonable.
I just checked Archive.org, to see what the dates were on one of my old toys.
I wrote a wrapper for the IE control, using an MDI interface, somewhere between 1999 and 2000. Though it didn't use tabs it was the first time I'd seen multiple documents in a single process.
Looks like I was late to the party! (Sadly the screenshot on the archived webpage is broken..)
I remember tabbed browsing showing up in either Galeon or Skipstone, well before Mozilla supported these. My recollection is that Opera actually supported the feature first. And though I thought it was earlier, the feature seems to have emerged around 2003, well after the date (1997) given in this article.
The 1995 date in the article definitely preceeds 1999, but the path to getting it into Mozilla does not seem correct.