A Study Evaluating the Efficacy and Safety of Mosunetuzumab in Combination With Lenalidomide in Comparison to Rituximab in Combination With Lenalidomide With a US Extension of Mosunetuzumab in Combination With Lenalidomide in Participants With Follicular Lymphoma

Inclusion Criteria: * Eastern Cooperative Oncology Group (ECOG) Performance Status of 0, 1, or 2 * Histologically documented CD20+ FL (Grades 1-3a) * Requiring systemic therapy assessed by investigator based on tumor size and/or Groupe d'Etude des Lymphomes Folliculaires (GELF) criteria * Received at least one prior systemic lymphoma therapy, which included prior immunotherapy or chemoimmunotherapy * Availability of a representative tumor specimen and the corresponding pathology report at the time of relapse/persistence for confirmation of the diagnosis of FL. Pretreatment sample of at least 1 core-needle, excisional or incisional tumor biopsy is required. Cytological or fine-needle aspiration samples are not acceptable. Fresh pretreatment biopsy is preferred. Patients who are unable to undergo biopsy procedures may be eligible for study enrollment if an archival tumor tissue sample (preferably from the most recent relapse/persistence) as paraffin blocks or at least 15 unstained slides, or in accordance with local regulatory requirements, can be sent to the Sponsor. * Adequate hematologic function (unless due to underlying lymphoma, per the investigator) * Agreement to comply with all local requirements of the lenalidomide risk minimization plan, which includes the global pregnancy prevention program. * For women of childbearing potential: Agreement to remain abstinent (refrain from heterosexual intercourse) or use 2 adequate methods of contraception, including at least 1 me…