Do Mormons believe all other religions are based on false doctrine?
No, Mormons do NOT believe that all other religions are based on false doctrine. They believe that elements of the truth can be found in a number of faiths. The do believe that the most truth is to be found in The Church of Jesus Christ of Latter-day Saints. Mormons do not condemn others to hell for not believing as they do, which sets them apart from some denominations. But they do believe that the greatest and highest blessings in eternity require accepting what they believe to be the restored church and living faithfully for the duration of their lives. All mankind, of whatever belief, will be rewarded according to their willingness to accept Christ and come unto Him, whether it be in this life or the next, prior to the final judgment.
I wouldn’t say that all churches besides ours are “based on false doctrine”–that would be extreme to say. We believe that most if not all religions do have origins in truth. However, we do believe that man’s philosophies and incorrect doctrines are mixed into some beliefs. For example, Christians–whatever denomination–do believe in Jesus Christ being our Redeemer, we don’t consider accepting him the Savior as false doctrine.