The participants of this study would have relapsed/refractory follicular lymphoma. Follicular lymphoma is a type of blood cancer. It is referred to as 'relapsed' when the disease has come back after a period of improvement after that follows a treatment regimen and 'refractory' when treatment no longer works. Stage 1 of this trial will study the safety and the level that adverse effects of each of the study drug combinations can be tolerated (known as tolerability). It is also designed to establish a recommended study drug dosage for stage 2 and 3. Stage 1 of the study is completed. Stages 2 and 3 will evaluate and compare how long participants live without their disease getting worse when receiving the study drug in combination with other drug treatment versus the placebo (dummy drug) in combination with other drug treatment.

Stage 1 is a safety run-in phase, was designed to evaluate the safety of the combination of tazemetostat and R2, as well as to establish the RP3D for Stage 2, which is now completed. Stage 2 is an efficacy and safety phase for an assessment of the FL population with the enhancer of zeste homolog 2 (EZH2) gain-of-function (GOF) mutation (EZH2 mutant-type \[MT\]) and without the EZH2 GOF mutation (EZH2 wild-type \[WT\]). In Stage 2, EZH2 WT and EZH2 MT patients will be randomly assigned in a 1:1 ratio to tazemetostat + R2 or placebo + R2. There will be 1 futility interim analysis (IA) and 1 efficacy IA for WT population and 1 efficacy IA for MT population. Stage 3 is a long-term follow-up of patients for assessment of response and overall survival. All patients will be followed for survival until 5 years post last patient enrolled in the study.