Popper's theory of knowledge is called critical rationalism, not falsificationism as explained by Popper in the introduction to "Realism and the Aim of Science". According to critical rationalism, knowledge is created by noticing problems, guessing solutions to those problems and then criticising the guesses to eliminate the ones that don't work. Popper's position is that neither guesses nor criticism can be justified in any way so they don't have to be justified. Rather, ideas are accepted or rejected as a result of whether they have survived criticism and nothing else, see "Realism and the Aim of Science" Chapter I and "Objective Knowledge" Chapter 1.
Doing an experiment is one way of criticising an idea, but it's not the only type of criticism. You can criticise an idea by thinking about experiments you could do that would lead to results that contradict some existing knowledge.
You can also motivate a guess at a solution in any way you like. That motivation might include imagining an experiment you don't actually perform - a thought experiment.