• Eastern Cooperative Oncology Group (ECOG) Performance Status of 0, 1, or 2

• R/R FL after treatment with at least one prior systemic lymphoma therapy, which includes prior immunotherapy or chemoimmunotherapy

• Previously untreated participants with FL must require systemic therapy assessed by investigator based on the Groupe d'Etude des Lymphomes Folliculaires (GELF) criteria and have a Follicular Lymphoma International Prognostic Index (FLIPI) score of 2-5

• Histologically documented FL of Grade 1, 2, or 3a, and that expresses CD20 at time of diagnosis as determined by the local laboratory

• Fluorodeoxyglucose avid lymphoma (i.e., positron emission tomography (PET) positive lymphoma)

• At least one bi dimensionally measurable nodal lesion (\>1.5 cm in its largest dimension by PET- computed tomography (CT) scan), or at least one bi dimensionally measurable extranodal lesion (\>1.0 cm in its largest dimension by PET-CT scan)

• Availability of a representative tumor specimen and the corresponding pathology report for confirmation of the diagnosis of FL

• Adequate hematologic function (unless due to underlying lymphoma, per the investigator) as defined by the protocol

• Negative HIV test at screening, with the following exception: Individuals with a positive HIV test at screening are eligible provided they are stable on antiretroviral therapy for at least 4 weeks, have a CD4 count ≥ 200/mL, have an undetectable viral load, and have not had a history of opportunistic infection attributable to AIDS within the last 12 months

• Normal laboratory values (unless due to underlying lymphoma) as defined by the protocol

• Agreement to comply with all local requirements of the Len risk minimization plan

• For women of childbearing potential: agreement to remain abstinent or use two adequate methods of contraception, including at least one method with a failure rate of \< 1% per year, for at least 28 days prior to Day 1 of Cycle 1, during the treatment period, and for at least 12 months after the final dose of glofitamab, 28 days after the last dose of Len, 18 months after the last dose of G, 3 months after the final dose of tocilizumab, and 3 months after the final dose of Mosun. Women must refrain from donating eggs during this same period

• For men: agreement to remain abstinent or use contraceptive measures and agreement to refrain from donating sperm, with female partners of childbearing potential or pregnant female partners, men must remain abstinent or use a condom during the treatment period and for at least 2 months after the final dose of glofitamab, 28 days after last dose of Len, 18 months after the last dose of G, 3 months after the final dose of tocilizumab, and 3 months after the final dose of Mosun