Random off road trails and bad sidewalks shouldn't just be exploding tires if they have enough pressure. I commute all over with my Surly Steamroller and no matter if I'm using 25c/28c/32c my tires never immediately burst. If you want a FGFS bike that fits tires the same size as your mountain bike a lot of companies have stopped making them but you can probably still find some around local ads like craigslist and ebay. If you want a FG that has road bike geo that fits bigger tires like REDMASTA suggested Kilo WT's from bikesdirect and Surly Steamrollers are popular. I think there's a FGFS section on this forum, you can go there and ask what's out. But a lot of the models I knew of with stock left were all cleared out in terms of complete fgfs bikes. You can still get Volume Thrasher fgfs framesets on ebay though if you wanted to build your own.