Foamed concrete (FC) offers low density and excellent thermal insulation. However, its structural application is limited by low strength and brittle failure caused by a highly porous internal structure. This study proposes a sustainable modification strategy for FC by utilizing waste seashell aggregates (cockle and oyster shells) as partial replacements for fine aggregates, combined with recycled fishing-net fibers (PP/PE/PA) as reinforcement. The microstructure–property relationships governing thermo-mechanical performance are systematically investigated using three-dimensional X-ray microcomputed tomography (micro-CT) and scanning electron microscopy (SEM). FC mixtures were designed at target dry densities of 600, 1000, and 1400 kg/m3, with seashell replacement ratios of up to 40%. Compressive strength, flexural performance, and thermal conductivity were experimentally evaluated, while micro-CT provided quantitative pore descriptors including porosity, sphericity, directional orientation tensors, and chordlength distributions. The results show that seashell aggregate incorporation generally reduces thermal conductivity while maintaining or improving compressive strength at comparable densities, consistent with matrix densification and enhanced interfacial transition behavior observed in SEM images. The inclusion of recycled fishing-net fibers significantly improves flexural ductility and toughness through crack-bridging and deflection mechanisms and further decreases thermal conductivity by disrupting pore connectivity. When shells and fibers are combined, synergistic benefits are achieved within intermediate replacement and dosage ranges, whereas excessive modification increases porosity and weakens matrix continuity, leading to strength reduction. Micro-CT-based correlations reveal that thermal conductivity is primarily governed by pore volume fraction and connectivity, while mechanical performance is strongly influenced by pore sphericity and orientation-induced anisotropy. Overall, this study demonstrates that discarded seashells, complemented by recycled marine fibers, can be effectively valorized as sustainable constituents in foamed concrete, providing a viable pathway for high-value recycling of marine waste in advanced construction materials.