Hello Mainul ! There is a function for grouping the tabs. First go to the menu and click on customize. There is an icon of tab grouping, drag it to your required position. When you click on it, you will see all the tabs in a box. Drag one tab out of the box to create a group. You can add as many tabs you want in the group. You can also give a name to the group. When you click one tab from the group the rest of the tabs on that group will show up in the top.

There isn't any icon such as that in customization. is something wrong?

If there isn't any icon then you can search for an add on feature which serve similar purpose.

thank you for helping

There's no such a feature in Firefox, but you can use this extension:

• https://addons.mozilla.org/firefox/addon/simple-tab-groups/

or one of those:

• https://addons.mozilla.org/firefox/addon/panorama-tab-groups/
• https://addons.mozilla.org/firefox/addon/basic-panorama/
• https://addons.mozilla.org/firefox/addon/conex/
• https://addons.mozilla.org/firefox/addon/power-tabs/
• https://addons.mozilla.org/firefox/addon/panorama-view/
• https://addons.mozilla.org/firefox/addon/tiled-tab-groups/