Glass fibre manufacturing plant comprising a forehearth forming a passage for conveying molten glass and defined by: - a first and second opposite longitudinal walls having a hot longitudinal wall surface facing said passage and extending along a longitudinal direction, X1, having a longitudinal wall thickness extending along a first transverse direction, X2, normal to X1, and having a longitudinal wall height extending along a second transverse direction, X3, normal to both X1 and X2, - a ceiling, - a bottom floor and - an end wall, wherein each longitudinal wall is made of a refractory masonry comprising: (a) a series of refractory base bricks forming a base wall comprising a top surface for supporting, (b) two spacer bricks lying on the top surface of the base wall and separated from one another at the level of the hot longitudinal wall surface by a distance, Wc, measured at room temperature along the longitudinal direction, X1, each spacer brick having a cuboid geometry comprising a hot surface forming a portion of the hot longitudinal wall surface, wherein, (c) a lintel of length, WL > Wc, measured along the longitudinal direction, X1, at room temperature and comprising two opposite ends, each resting on one surface of one of the two spacer bricks, thus defining with the top surface of the base wall and the two spacer bricks, (d) a cavity of width, Wc, and height, H1c, measured at the level of the hot longitudinal wall surface at room temperature along the longitudinal direction, X1, and along the second transverse axis, X3, respectively, and of depth, Dc, measured at room temperature along the first transverse axis, X2, (e) a support block comprising a hot cuboid portion having a cuboid geometry of width, w, and height, h, measured along X1 and X3, respectively, at the level of the hot longitudinal wall surface at a service temperature, hT, of the forehearth of at least 1000°C, and of depth measured along X2 at least equal to D, wherein w < Wc, and h < H1c, said hot cuboid portion being reversibly inserted in the cavity, and (f) a gap surrounding the hot cuboid portion of the support block when positioned in the cavity, said gap being filled with a resilient material, characterized in that, the masonry comprises a spacing element hindering the thermal expansion of the two spacer bricks, such that the distance, Wc, between said two spacer bricks measured at room temperature along the longitudinal direction, X1, at the level of the hot longitudinal wall surface cannot be reduced below a predetermined hot cavity width, W, at said service temperature, hT, wherein said predetermined distance, W, is larger than the width, w, of the hot cuboid portion of the support block. - a first and second opposite longitudinal walls having a hot longitudinal wall surface facing said passage and extending along a longitudinal direction, X1, having a longitudinal wall thickness extending along a first transverse direction, X2, normal to X1, and having a longitudinal wall height extending along a second transverse direction, X3, normal to both X1 and X2, - a ceiling, - a bottom floor and - an end wall, (a) a series of refractory base bricks forming a base wall comprising a top surface for supporting, (b) two spacer bricks lying on the top surface of the base wall and separated from one another at the level of the hot longitudinal wall surface by a distance, Wc, measured at room temperature along the longitudinal direction, X1, each spacer brick having a cuboid geometry comprising a hot surface forming a portion of the hot longitudinal wall surface, wherein, (c) a lintel of length, WL > Wc, measured along the longitudinal direction, X1, at room temperature and comprising two opposite ends, each resting on one surface of one of the two spacer bricks, thus defining with the top surface of the base wall and the two spacer bricks, (d) a cavity of width, Wc, and height, H1c, measured at the level of the hot longitudinal wall surface at room temperature along the longitudinal direction, X1, and along the second transverse axis, X3, respectively, and of depth, Dc, measured at room temperature along the first transverse axis, X2, (e) a support block comprising a hot cuboid portion having a cuboid geometry of width, w, and height, h, measured along X1 and X3, respectively, at the level of the hot longitudinal wall surface at a service temperature, hT, of the forehearth of at least 1000°C, and of depth measured along X2 at least equal to D, wherein w < Wc, and h < H1c, said hot cuboid portion being reversibly inserted in the cavity, and (f) a gap surrounding the hot cuboid portion of the support block when positioned in the cavity, said gap being filled with a resilient material,