Making his first starting appearance since December, the Scotland striker scored his first goal at Celtic Park in ten months as his rehabilitation from a potentially career-threatening bout of depression reached a landmark moment.Griffiths found the net with a stunning free-kick at the end of the first half as Celtic swept Estonian champions Nomme Kalju aside to make the second leg of this second qualifying round tie in Tallinn next Tuesday little more than a formality.The outstanding Ryan Christie claimed a double, while Kristoffer Ajer and Callum McGregor, pictured, were also on target for Celtic. But this was Griffiths’ night and his emotionally-charged celebration of his goal was shared with pure joy by the home fans. He left the field to a standing ovation when he was replaced by Lewis Morgan just before the hour mark.It wasn’t simply his magnificent free-kick which signalled that Griffiths is assuredly ready to resume his Celtic career in the most effective and profitable manner possible.His overall performance suggested that he is not only conquering his mental demons, he is recovering the kind of physical condition required to operate at the kind of level the champions demand of him.Playing alongside Odsonne Edouard as manager Neil Lennon opted for a 3-5-2 formation, Griffiths was full of energy as he looked to stretch a Kalju defence which, for the first half hour at least, was defiantly robust.It’s not only Griffiths’ goals Celtic have missed, it is his delivery from set-pieces. He created their first chance of the night in the sixth minute with a superbly whipped in corner from the right which Jozo Simunovic headed just wide at the far post.Lennon’s side were as dominant in possession as anticipated but had to exercise patience in their quest for a breakthrough. Kalju goalkeeper Pavel Londak made the first of several saves in the tenth minute when he reacted well to divert a stinging left-foot shot from Christie over the top.The visitors’ priority was clearly to try and frustrate Celtic for as long as possible but they carried an occasional attacking threat of their own on the counter. Brazilian striker Liliu looked the most likely source of any anxiety for Celtic and he almost provided Kalju with a shock lead when he wriggled free down the right and sent in a fine cross which Peeter Klein should have done better than head wide from close range.Kalju were soon hemmed into their own half again as Celtic upped the tempo. Edouard was unfortunate to see his close-range header from another Griffiths corner rebound off team-mate Simunovic as chances began to come Celtic’s way regularly.Griffiths, understandably keen to get that long-awaited goal, was just too eager when he sliced wide from a James Forrest cross. He then saw his first free-kick attempt of the night break off the defensive wall into the path of Scott Brown whose shot was well saved by Londak.Kalju’s resistance was finally broken in the 37th minute. Edouard was crudely fouled by Andriy Markovych, earning a free-kick which Christie swept over from the left for Ajer to force home a close range header via the underside of the bar.Celtic had to make an early substitution, left wing-back Boli Bolingoli limping off for the second time in his three appearances so far. He was replaced by Mikey Johnston.The home side now had the bit firmly between their teeth and they doubled their lead in the 44th minute. Christie’s tenacity forced Kalju defender Aleksandr Kulinits to rashly flick out an arm at the ball as he tried to clear inside the area. After brief hesitation, Danish referee Jakob Kehlet pointed to the spot. Christie stepped up and sent Londak the wrong way as he smashed home the penalty.The icing on Celtic’s first-half cake came three minutes into stoppage time with a moment Griffiths will cherish for some time to come. Vladimir Avilov’s clumsy foul on Christie conceded the free-kick which was instantly reminiscent of the territory from where Griffiths scored his sensational double for Scotland against England at Hampden two years ago.It was almost a carbon copy of one of those strikes as Griffiths, following an expectant hush around the stadium, bent his left-foot effort over the wall and beyond the despairing left hand of Londak into the corner of the net. Kalju appeared increasingly dispirited as Celtic emphasised their superiority and showed no signs of easing off after the break. But for the continued excellence of Londak, the visitors would have fallen further behind long before Christie did claim Celtic’s fourth goal in the 65th minute.After substitute Johnston’s surge into the penalty area was halted, the ball broke to Christie on the right and he curled a superb left-foot shot around Londak into the far corner.The tie was effectively over already but Kalju’s agony continued. Celtic made it 5-0 in the 77th minute, substitute Olivier Ntcham breaking forward to set up McGregor for a shooting chance which he swept beyond Londak from 20 yards.